Relying on relational capital theory and transaction cost economics (TCE), this study identifies factors that impede or promote alliance formation in small to medium-sized enterprises (SMEs). Environmental uncertainty and knowledge intensity impede firms' R&D alliance formation; the focal firm's overall trust in partners enhances alliance formation. Trust interacts positively with environmental uncertainty and knowledge intensity to affect alliance formation in SMEs. The findings reflect data from a longitudinal sample of 854 German SMEs, captured over eight years from 1999 to 2007. (C) 2012 Elsevier Inc. All rights reserved.