Kinetics of thermal decomposition of C-10-C-14 n-alkanes and their mixtures was studied under near-critical and supercritical conditions. Supercritical-phase thermal decomposition of n-alkanes can be represented well by an apparent first-order kinetics, even though the decomposition was not a true first-order process. A generalized expression was developed to predict the apparent first-order rate constants for the decomposition of C-8-C-16 n-alkanes at 425 degrees C. Pressure had a significant effect on the apparent first-order rate constant in the near-critical region. This large pressure effect can be attributed to the significant changes in density and possibly to the changes in the rate constants of elementary reactions with pressure in this region. Individual compounds interacted with each other in the thermal reaction of n-alkane mixtures. The overall first-order rate constants for n-alkane mixtures can be predicted satisfactorily from the rate constants for pure compounds.