Introduction The fashion business is known as one of the major industries that is suffering from rising concerns about the consumption of its product, which led to a reorganization of the fashion supply chain to become more sustainable three decades ago. The interest in the concept of sustainability and demand for sustainable marketing activities is gradually growing in the fashion industry due to the negative image and press it receives. Within the luxury fashion segment, the three main themes that are recognized to contribute to sustainability are exclusivity, craftmanship and limited production. However, luxury brands are increasingly shifting their attention and commitment towards environmental and social issues to be incorporated in the concept of sustainability. Yet, the majority of consumers has little understanding or misunderstands the concept of sustainable fashion and marketing, which leads to a gap between attitudes towards sustainability and actual behavior. As a result, fashion brands are trying to leverage their brand by making sustainability a key marketing strategy to raise awareness about social, environmental, economic and cultural issues. Extant research has not explored this recent trend to understand how consumers evaluate fashion brands with a sustainable marketing communication, especially in the context of luxury brands. This study investigates how luxury and mass fashion brands can utilize sustainable marketing contents in social media communication to reach their target group and enhance their equity with sustainability associations. Theoretical Development Associative network models of memory have served as a fundamental framework for a wide range of studies related to the formation and transfer of associations. According to associative network theory, brand knowledge is represented in form of an associative network of memory nodes connected to each other. Nodes are activated when cues, such as advertising, are presented. Mere exposure to cues was shown to be sufficient to active associations and facilitate association transfer. While brands are continuously attempting to make use of associative power to leverage brand equity, extant research has provided compelling reasons to accept that association transfer can also result in brand dilution when a retrieval of conflicting or negative associations occurs. Especially in the context of luxury brands consisting of very unique associations and being different from mass brands in many regards, managing the brand’s associative network is a crucial task in order to send the right signals to consumers and maintain exclusivity. This study investigates how social media communication of different sustainability dimensions affects brand attitude and how it ultimately impacts behavioral outcomes in an attempt to build brand equity for mass and luxury fashion brands. Method and Data The hypotheses are tested with 273 respondents who participated in an online experiment. They were first asked to state their involvement with the category fashion. Subsequently, subjects were presented with a brand post either for the mass or luxury brand including claims related to one of the four sustainability dimensions or no claims for the control group respectively. The experiment consisted of a 2 (brand: mass or luxury) x 5 (sustainability dimensions: none, cultural, economic, environmental, social) factorial design. The measures that followed included attitudinal as well as behavioral constructs related to the brand, sustainability as well as social media use. Analysis of covariance is applied to test for main effects and interaction effects. Summary of Findings This study provides evidence that social media communication of a sustainable brand affects the purchase intention of consumers. The findings indicate a significant difference between the mass and the luxury brand used for this study. The mass brand exhibits the potential to leverage associations with cultural, economic, and environmental sustainability. However, the results only reveal a marginally significant higher purchase intention when cultural sustainability is communicated compared to when the brand does not provide any sustainable associations. In contrast, the luxury brand suffers from significant brand dilution across all four sustainability dimensions resulting in a decline in purchase intention. Key Contributions The findings reveal that sustainability communication exerts a diverging influence depending on the type of brand that is involved. This study suggests that mass brands are able to benefit from sustainability communication in an attempt to leverage brand equity. However, for a luxury brand this type of associations rather presents a liability that might dilute the brand. The findings of this study provide important insights for brand managers. Since mass brands are currently increasing efforts into sustainable communication in the fashion industry, the results suggest that this might be a promising investment. However, luxury brands are advised to carefully manage the communication of salient content related to sustainability as it might harm the invaluable and unique associations inherent in a luxury brand.