Abstract The effector domain (ED) of MARCKS proteins can associate with calmodulin (CaM) as well as with phospholipids. It is not clear, however, whether a complex between MARCKS proteins and CaM can form at the surface of phospholipid membranes or whether CaM and membranes compete for ED binding. Using two-mode waveguide spectroscopy, we have investigated how CaM regulates the association of MARCKS-related protein (MRP) with planar supported phospholipid bilayer membranes. Bringing a solution containing CaM into contact with membranes on which MRP had previously been deposited results in low-affinity binding of CaM to MRP. A preformed, high-affinity CaM · MRP complex in the aqueous phase binds much more slowly than pure MRP to membranes. Similar observations were made when a peptide corresponding to the ED of MRP was used instead of MRP. Hence CaM cannot form a stable complex with MRP once the latter is bound at the membrane surface. CaM can, however, strongly retard the association of MRP with lipid membranes. The most likely interpretation of these results is that CaM and the phospholipid membrane share the same binding region at the ED and that the ED is forced by membrane binding to adopt a conformation unfavorable for CaM binding.