This study responds to a call to explain recent outbreaks of cyber-bullying on mobile instant messengers (MIMs). We examined the relationships between MIM bullying opportunities and bullying behaviours in group-chat settings while also investigating the moderating effect of adolescents' motivations for MIM use. We gathered survey data from a sample of 417 South Korean students aged 13-17 years. The results indicate that adolescents' perceptions of opportunities for cyber-bullying on MIMs are positively associated with their levels of engagement in bullying behaviours in MIM use. Additionally, we found a significant moderating effect of two motivations for MIM use, entertainment and convenience. We discuss the personal and social implications of our findings that engaging in cyber-bullying is affected by user perceptions and the types of gratification offered by a given medium.