Termination kinetics in tert-butyl methacrylate (tert-BMA) and n-butyl methacrylate (n-BMA) bulk homopolymerizations has been studied via the single pulse-pulsed laser polymerization-near infrared (SP-PLP-NIR) method between 40 and 80 °C at pressures from 500 to 2 250 bar. Toward increasing monomer conversion, the chain-length averaged termination rate coefficient, <kt>, for both monomers exhibits the methacrylate-specific sequence of an initial plateau region, assigned to control by segmental diffusion, followed by a steep decrease of <kt> at intermediate conversion, which is assigned to translational diffusion control, and a weaker decrease of <kt>, associated with reaction-diffusion control, at still higher degrees of monomer conversion. Despite this similarity, the two isomeric monomers clearly differ in absolute size of <kt> and in the monomer concentration ranges where the transitions between the different types of diffusion control occur. The differences are assigned to effects of chain mobility which is hindered to a larger extent in tert-BMA than in n-BMA. As a consequence, the <kt> behavior of tert-BMA at 80 °C is close to the one of n-BMA at 40 °C. Investigations into the chain-length dependence of kt, in particular into kt(i,i), the rate coefficient for termination of two radicals of identical size, support the evidence on the different types of diffusion control that operate as a function of monomer conversion. In the initial conversion range, the power-law exponent which characterizes the chain-length dependence of larger (entangled) radicals, is found for both monomers to be close to the theoretical value of α = 0.16.