For a long time research on the management of IS outsourcing projects viewed relational and con-tractual governance as substitutes. However, subsequent studies provided empirical evidence for the complementary view. Recently, some authors supported the notion that relational and contractual governance mechanisms can simultaneously be complements and substitutes. Given these inconsistencies the question arises as to how contractual and relational governance be-come substitutes or complements. We investigate, whether the relationship between governance me-chanisms is the outcome of distinct processes of interaction between contractual and relational gover-nance. For that purpose, we conduct an exploratory, multiple-case study of five IS outsourcing projects at a leading global bank. We identify three archetypical processes illustrating how the interaction between relational and con-tractual governance can result in a complementary relationship. In addition, we discover one process explaining their substitution. The results of our study propose a shift in perspective. While former studies focused on explaining, whether contractual and relational governance are complements or substitutes, we answer the ques-tion how and why they become complements and substitutes. Based on our findings, we give implica-tions for further research.