The free-radical polymerization of butyl acrylate is studied between 25°C and 80°C at pressures up to 2500 bar. The reaction is measured by applying excimer laser techniques in conjunction with time-resolved high-pressure near infrared spectroscopy. Rate coefficients of propagation, kp, and termination, kt, are obtained for an extended temperature, pressure, and conversion range. The dependence of kt on conversion is analyzed by means of a kinetic model which considers segmental diffusion, translational diffusion, and reaction diffusion. It turns out that free-radical termination is essentially determined by reaction diffusion.