Although many studies employ social network theory to explain firm innovation, how individual-level factors lead to a firm's collective innovation capability remains under-researched. Building on studies that use the work boundary to define formal and informal social interactions, this research aims to illuminate how informal buyer-supplier employee interactions influence buyer firms' innovation capabilities through knowledge acquisition. Integrating the literatures on absorptive capacity and social interaction, the analysis of survey data from 273 Chinese manufacturing firms suggests that employees' informal interactions are positively associated with knowledge acquisition and enhance firms' innovation capabilities. Furthermore, the indirect effects of informal interactions on innovation capability are moderated by knowledge application.