The decomposition of di-tert-butyl peroxide (DTBP) in n-heptane with varying amounts of pivalic acid (PA) being added was studied via online FT-IR spectroscopy at elevated temperatures, 180 to 200 degrees C, and pressures up to 2000 bar. A pronounced acid-induced DTBP decomposition is found without any PA consumption. Decomposition rates are adequately described by first-order kinetics. The pressure and the temperature dependence of acid-induced decomposition are investigated. The data suggest that the rate enhancement is due to the formation of a DTBP-PA complex. The acid-induced DTBP decomposition yields the same products as does the non-induced process (without acid being present). It thus appears justified to use the rate data observed for the model system DTBP-PA-n-heptane for the simulation of initiation rates in technical polymerizations where DTBP acts as the initiator and where acid is present, e.g. as a monomeric species.