The purpose of this study is twofold. Firstly, the paper calls attention to a unique subset of professional service providers (PSPs) who have obvious product expertise related to their profession yet are only minimally involved in the sales process. Despite this, they are considerably influential in regard to their clients’ brand decision-making process. Examples of these include veterinarians recommending a given pet food brand and dentists recommending a brand of dental floss. Secondly, since environmental sustainability is an important facet of corporate social responsibility, we considered factors that help predict PSPs’ propensity to recommend environmentally friendly brands to their clientele. The conceptual model argues that PSPs’ degree of identification with their profession, self-assessed knowledge in environmental concerns and expectation that sustainable brands will engage in eco-based marketing are all positive influences on PSPs’ likelihood to recommend sustainable brands and generate positive word of mouth. Responses from 467 veterinarians, representing this unique class of PSPs were analysed, and structural equation modelling determined that identification with the profession was a motivating factor in learning more about sustainability. In turn, self-assessed expertise in environmental issues was shown to increase the likelihood that PSPs would recommend sustainable brands.