This paper investigates social media usage, focusing on the association between the influencer’s credibility and purchase intention. Building on the theory of source credibility and involvement inventory, a mediating effect research framework is proposed and evidenced. To test our proposed framework, data were collected via on-line survey and analysed by using partial least squares structural equation modelling (PLS-SEM). Drawn from 254 Thai social media users, our study suggests that credibility has a significant association with cognitive and affective responses as well as with normative and informative social influences. Upon which credibility has the most influence on affective response. To this end, our data indicate that social media followers increase their purchase intention through cognitive response. Nevertheless, social influences do not seem to have an inter-relationship with personal responses and do not strengthen the relationship between responses and purchase intention. These results point to several important theoretical implications and empirical advice to practitioners.