Vesicle populations having narrowly distributed size distribution are mixed with each other to investigate the extent and degree of mutual interaction. One basic question is whether these vesicular systems can be seen as a "kinetic trap" or whether they reequilibrate with each other to form a common energy minimum. To this aim, different size distributions of oleate and POPC vesicles (POPC stands for 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) were prepared by extrusion methods. Accordingly, three different systems were investigated: POPC/POPC vesicles; oleate/oleate vesicles; POPC/oleate vesicles. Dynamic light scattering technique was used to follow the possible size changes in aggregation after the mixing of the two vesicles populations. In agreement with the "kinetic trap" notion, no interaction was observed for the first two systems; i.e., the two populations of vesicles coexisted for several days without change of their original size distribution. In the case of the oleate/oleate system, addition of Call ions brought about the turbidity increase, indicating an aggregation between the negatively charged oleate vesicles. These changes were however completely reversible, as shown by the addition of excess EDTA, which can be taken as evidence that the observed changes were not due to fusion but to a simple transient aggregation. For the system of POPC/oleate, the two peaks of the two different vesicles populations rapidly merged into only one peak, due to the disappearance of the oleate peak. This was explained by the uptake of monomeric oleate by POPC vesicles, mediated through the relatively large critical aggregate concentration (cac) of oleate vesicles. Finally, the oleate vesicles of different size were exploited to study the different rate of uptake of added surfactant monomer. It is shown that the larger vesicles are faster, even when care is taken of normalizing to the same overall surface.