As Green IT is a relatively new area in Information Systems research, the first wave of research has often focused on organisations’ adoption of Green IT. In this study, we look beyond the initial adoption and investigate the assimilation of Green IT by organisations. We draw from and compare two theories – contextual theory and absorptive capacity – and investigate which of the two theories better explains the level of Green IT assimilation. Results from an international survey of 148 large organisations show that both theories explain Green IT assimilation, however while contextual theory has a medium to large effect, absorptive capacity has a small to medium effect.