The mechanism of charge-transfer complexation in electron-donor (D)/electron-acceptor (A) active layer was studied for a pseudobinary blend model system, poly(4-vinylpyridine) (P4VP)/[6,61-phenyl-C-61-butyric acid methyl ester (PCBM) in DMF solution, by a combination of light scattering (LS), transmission electron microscopy (TEM), and UV-vis spectroscopy. The time evolution of the system can be characterized by four distinct stages, i.e., induction, complexation, aggregation, and precipitation. In the induction stage, a combined dynamic LS and static LS studies showed that the conformation of P4VP remained unchanged, while the UV-vis indicated that the charge-transfer complexation had almost accomplished with an obvious broadening at C-60 characteristic absorption peak of 330 nm. In the complexation stage, each P4VP chain complexed with about three PCBM molecules at c(p4vp) = 4.1 x 10(-2) g/mL, C-PCBM = 5.8 x 10(-4) g/mL, and the molar ratio [4VP]/[PCBM] = 57:1 and shrinked in size with almost no change in the UV-vis spectrum. In the subsequent aggregation stage, P4VP/PCBM complexes aggregated with each other to form spherical aggregates with again unchanged UV-vis signals. The free association model (FA model) can be used to explain this mechanism. In the final precipitation stage, huge P4VP/PCBM agglomerate began to phase out with a clear decrease of the scattered light intensity in the LS. The almost unchanged UV-vis spectrum after the induction stage proved that the electronic transition from ground to excited state is not necessarily to be influenced by any inter- or intrapolymer structural transition. Our kinetics study on the mechanism of the complexation/association/aggregation shed some light on the possible morphology control in the D/A active layer.