The high temperature oxidation reactions of acetylene and methane have been studied in shock waves using two technics. A time-of-flight mass spectrometer has been used to determine the concentration of reactants, intermediates, and products at 50 μsec intervals throughout the reaction. In another apparatus the chemiluminescent radiation emitted by CH and C2 and the ionization accompanying the oxidation have been monitored.The oxidation of acetylene proceeds by fast chain branching reactions. Most of the observations presented in this paper are well explained by the chain branching mechanism: H+O2→OH+O (1) O+C2H2→OH+C2H (2) OH+C2H2→C2H+H2O (3) C2H+O2→2CO+H (4) C2H+O2→CO2+CH (5) C2H+C2H2→C4H2+H (6) CH+O2→CO+OH. (7) Reaction (1) is the slowest reaction and thus controls the rate of chain branching.The oxidation is accompanied by chemi-ionization. The C3H3+ ion is observed in abundance early in the reaction and we believe that under our conditions this species is the first ion to be formed. A cyclic structure, having stability derived from delocalization of its π electron system, is proposed for this ion. The mechanism of chemiluminescence is also discussed.The oxidation of methane is characterized by an induction period which is followed by fast chain branching reactions. The duration of the induction period is controlled by the rate of production of oxygen atoms. A mechanism involving a straight chain reaction to produce an intermediate which itself takes part in chain branching reactions is suggested.