This study evaluates the effect of pre-existing relational governance mechanisms on environmental collaboration. Specifically, our study distinguishes between structural and process dimensions of relational governance mechanisms so as to facilitate a more nuanced investigation into the inherent complementarities and performance implications. Using data collected from 145 US firms and a combination of methodologies - three-stage least squares and structural equation modelling, a number of direct, complementary and mediation effects are hypothesised and tested. The three-stage least squares methodology was adopted to overcome endogeneity and simultaneity issues inherent in the hypotheses covering complementarity. Contrary to conventional wisdom, structural and process governance mechanisms were not found to act as complements for environmental collaboration. Instead, the effect of structural mechanisms was found to be completely mediated by the process mechanisms. Thus, process mechanisms of relational governance were found to be much more important in promoting advanced practices such as environmental collaboration. Our results also document the significant mediating role of environmental collaboration. Implications for future research and practice are offered.