As more consumers are interested in sustainability issues and evaluate brands based on their social responsibility issues involved, firms are using social media platforms in order to communicate with consumers about Corporate Social Responsibility (CSR) activities. However, the current literature focuses mostly on factors drive engagement of promotional messages, while little evidence was found in the literature on how to design effective CSR messages on social media to engage consumers. Using over 3,000 branded posts on Facebook and Twitter, this research explores factors that help CSR messages become more engaging in social media. The results show that, on average, CSR messages had a negative significant effect on consumer engagement. However, CSR messages became more engaging when designed with emotional appeals, longer texts, and a hashtag. While marginal, CSR messages with informational appeals and humor undermined the effect of CSR messages on engagement. Finally, we explore different types of CSR messages by their beneficiary scope and the role of brands in the message and discuss what message contents drive more engagement in social media. This research contributes to the academic literature and managers by providing new insights on how to design CSR messages for effective communications in social media.