This study examines the motives of employees to engage in work related social media use - i.e. the use of personal social media accounts to communicate about work-related issues. The theory of planned behavior (TPB) was used to explain this behavior. Because social media can enable users to express their identities, social identity expressiveness and self-identity expressiveness were added to the TPB model. Through an online questionnaire, using purposive sampling technique, 514 Dutch employees were asked about their social media use and motivation to do so. We used structural equation modelling (SEM) to test our hypotheses. Results indicate that these identity constructs enhance the predictive ability of the TPB. As such, work-relation social media use is likely to take place spontaneously rather than deliberately and consciously planned.