The social media accounts of celebrities have inspired innovative ways for companies to promote their brands. One form of celebrity-related advertising is celebrity-sponsorship posts. These posts often come at a high cost for the advertiser while it remains unclear which content characteristics are expected to influence the effectiveness of these posts. This paper investigates the impacts of content characteristics of celebrity-sponsorship posts on Facebook on customer engagement. Structural equation modelling was used to assess the characteristics of 313 sponsored posts from 128 celebrities endorsing 154 brands. Drawing on the dual mediation hypothesis and using gratification theory, this study reveals that distinct content characteristics drive either attitudes towards the celebrity or attitudes towards the brand, revealing the double-edged nature of celebrity-sponsorship posts. The findings suggest that marketing managers should carefully design celebrity sponsorship posts by adjusting the balance between celebrity and brand emphasis. Otherwise, there might be a threat of either the brand being overshadowed by the celebrity, thus reducing brand attitude, or the brand 'capturing' the celebrity by - from a fan's perspective - being obviously recognised as plain advertising; as such, this effect can harm the attitude towards the celebrity and, consequently, reduces customer engagement.