Thermal decomposition of C-10-C-14 n-alkanes was studied under near-critical and supercritical conditions. The primary products were C-1-C-m-2 n-alkanes and C-2-C-m-1 1-alkenes, and the secondary products were cis- and trans-2-alkenes, n-C-m-1, n-C-m+1, and C-m+2-C-2m-2 normal and branched alkanes, where m is the number of carbon atoms in the reactant. The relative yields of the primary and secondary products were dependent upon the reaction conditions. Product distributions exhibited large pressure dependence in the near-critical region. The observed product distributions and changes in product composition with reaction conditions were explained by a modified free radical mechanism.