A phenomenon of significance in buyer–supplier relationships is opportunism. In lieu of the known negative effects of opportunistic behavior on buyer–supplier relationships, the circumstances in which a sourcing professional engages in acts of opportunism are unclear. Combining theories from multiple disciplines, a comprehensive model tested buyer–supplier relationship-specific factors, environmental factors, an individual-difference factor, and situational factors likely to affect a buyer's decision to use opportunistic tactics. Results reveal how these different theories combine to provide amore comprehensive explanation of buyer behavior than existed in prior literature. Using structural equation modeling of a sample of 328 procurement transactions, factors found to affect buyer opportunism included buyer power, corporate ethical values, honesty/integrity, leader opportunism, willful ignorance, and subjective expected utility. This study also provides empirical support for distinguishing between two types of opportunism — strong and weak. The research concludes with implications for theory and practice, limitations, and areas for future research.