Copolymerization termination rate coefficients (kt) of the methyl methacrylate-dodecyl acrylate (MMA-DA)and dodecyl methacrylate-methyl acrylate (DMA-MA) systems at 40 °C and 1000 bar have been measured using the single pulse(SP)-pulsed laser polymerization (PLP) technique. Plateau regions of kt are observed in the initial polymerization period. The region of constant kt increases with the size of the alkyl ester group, e.g. it extends up to at least 60% monomer conversion in DA and DMA homopolymerizations. The plateau kt values of MA and MMA are significantly above the corresponding DA and DMA values. The kt penultimate unit effect model, which uses the so-called geometric mean approximation, is well suited for representation of the dependence on monomer composition of the plateau kt values. The dependence of kt on monomer composition is quite different at low and at high degrees of monomer conversion. The reason behind this is seen in different types of diffusion control being operative at low and at high degrees of monomer conversion. The plateau-type behaviour is assigned to segmental diffusion control, whereas high-conversion kt bis controlled by reaction diffusion.