Inter- and intrabilayer transfer of phospholipid molecules was investigated by neutron reflectometry. The structure of solid supported lipid bilayers exposed to a solution of isotopically labeled vesicles was monitored as a function of temperature, time, and vesicle concentration. Lipid interbilayer exchange was shown to be the time limiting process, while lipid intrabilayer movement, the so-called flip-flop, was too fast to be visualized within the experimental acquisition time. The exchange process was characterized by an Arrhenius-like behavior and the activation energy of the process was concentration-independent. The results are discussed and compared extensively with the literature available on the topic.