N-Vinylpyrrolidone polymerization photoinitiated at 365 and 546 nm by azidopentaammine cobalt(III) {[Co(NH3)(5)N-3](2+)} was investigated at room temperature in an argon atmosphere. By excitation into the ligand to metal charge transfer (LMCT), the cobalt complex showed an efficient photoredox process leading to the formation of a cobalt(II) and an azide radical (N-3, Phi(photoredox) = 0.24). The same process was found to occur by excitation into the ligand field band with a low but not negligible quantum yield (Phi(photoredox) = 0.016). Two different domains were clearly present when the plot of the rate of polymerization as a function of the cobalt(III) complex was studied; for [Co(III)] < 2.0 x 10(-4) M, the termination step mainly involved a mutual annihilation of growing radicals whereas an oxidative termination was present in the range of 2.0 x 10(-4) M < [Co(III)] < 1.0 x 10(-3) M. Within the former domain the rate of polymerization (R-p) varied with the first power of the monomer concentration and with the square root of the absorbed light intensity while for the latter domain the R-p was proportional to the monomer concentration and absorbed light intensity. Further investigations using the viscosity-average molecular weight data allowed us to corroborate the proposed polymerization mechanism.