The kinetics of the free-radical polymerization of isobornyl methacrylate (IBoMA) at 80 degreesC initiated by benzoyl peroxide (BPO), was determined by differential scanning calorimetry (DSC), using sample pans previously sealed in air or in nitrogen. The polymerization was arrested by vitrification at a final conversion close to 0.80. The kinetics in nitrogen could be fitted using initiation and propagation rate constants reported in the literature, a functionality of the termination rate constant controlled by translational diffusion based on the free-volume model, and an efficiency factor for the initiator decomposition that decreased with conversion in the medium-high conversion range. The presence of oxygen led to a significant decrease in the polymerization rate, an effect that was enhanced when decreasing the sample size (increasing the surface-to-volume ratio). The original finding was the presence of two maxima in the isothermal polymerization rate, a fact that was confirmed by replicating experiences in two different DSC devices and changing the amount of initiator in the formulation. The first maximum in the polymerization rate was explained by the decrease of the diffusion rate of macromolecular radicals to the oxygen-rich boundary, an effect that started at low conversions. The second maximum was related to the decrease in the solubility/diffusion of oxygen in the reaction medium, a phenomenon that was particularly severe at high conversions.