Informal networks between individuals are widely seen as important in East Asian business systems. However, while the performance implications of guanxi in China have been extensively studied, much less is known on how informal networks may influence business outcomes in other East Asian countries. We examine informal networks in inter-organizational research collaborations in South Korea by studying the role of pre-existing social ties and relational orientation in new product development (NPD) collaborations and university-industry research (UIR) collaborations. We conduct an interview-based, in-depth case analysis of five NPD collaborations and five UIR collaborations. Pre-existing social ties and relational orientation are prevalent in both types of research collaborations. However, they have different consequences for collaboration outcomes. Pre-existing social ties strongly vary in their types and strength, and negatively affect collaboration outcomes unless they result in the selection of competent and motivated partners. Relational orientation manifests itself in a task-related and social exchange between collaboration partners, which contributes to outcomes such as knowledge acquisition and business performance. Korean collaboration managers should rely only on pre-existing social ties that are well-aligned with task-related requirements when initiating and conducting research collaborations. However, the creation of strong relationships between collaboration team members of partnering organizations should be encouraged.