The bulk ring-opening polymerization (ROP) of ε-caprolactone (CL), catalyzed by molybdenum trioxide (α-MoO3), was investigated under nitrogen atmosphere at 150°C. Effect of monomer/initiator ([CL]0/[I]0, I = MoO3) and monomer/initiator/co-initiator ([CL]0/[I]0/[CoI]0, CoI = ROH and H2O) ratios on monomer conversion and molecular weight were determined. Bulk polymerization of CL was completed within 20 hours at 150°C for [CL]0/[I]0 = 100. Propagation was first order in CL concentration and initiator at this temperature, being the rate constant of propagation kp equal to 2.27 × 10−4 mol−1 l s−1. Polymerization reaction was accelerated by the addition of small amounts of water and n-octanol. In the presence of water (up to [CL]0/[H2O]0 = 120), polymerization rate increases and was completed within 6 hours at 150°C; for higher amounts of water (150 to 180), lower conversions are observed. However, control of number-average molecular weight was only efficient for [CL]0/[H2O]0 = 20 and 30. In the presence of n-octanol, degradation of polyester occurs at early stages of polymerization. Kinetic data for polymerization (obtained by 1H-NMR) were fitted to 14 different model reaction functions. It was found that a linear model represents better the conversion versus time plots for bulk polymerization, in agreement with the pseudo living nature of polymerization.