Vulnerability to environmental and climate change, and the measures taken to reduce or adapt to it, are already well researched. However there is little research on how climate change adaptation is being mainstreamed into the large development investments of major donors in developing countries. Multilateral Development Bank (MDB) investments are often in climate-sensitive sectors and locations, particularly those exposed to extreme events like river basins and low-lying floodplains and deltas. A donor's "as-usual" investments may actually increase climate change vulnerability or hamper the progress of poverty alleviation in the face of climate change. MDBs have recognized that efforts to overcome poverty and advance development can no longer ignore the urgent need to address climate change risks. With foresight, planning can be designed to reduce these risks. However, there is little evidence to show how MDBs have actually incorporated climate change adaptation (CCA) considerations into their development investments and how MDBs can become a guiding example for others (Boakye-Agyei, 2011, p. 265). Building on institutional theory associated with environmental impact assessment, this paper explores how climate change adaptation has been empirically mainstreamed into a Multilateral Development Bank-funded road infrastructure investment in Cambodia. The paper therefore provides a contribution to understanding whether the MDBs can play a leading role in promoting CCA considerations into their current and future development investments.