The oxidation of acetylene has been studied using sufficiently high gas densities and concentrations of inert gas to keep the reaction isothermal and reduce boundary-layer effects. The induction periods and exponential time constants of the oxidation have been measured using the observations of: (a) chemiluminescence and gas conductivity in incident and in reflected shock waves, and (b) total ionization and product formation using a time-of-flight mass spectrometer in reflected shock waves. The results are internally consistent, and the time constants τ are best represented by the following equation: log10(τ[O2])=−11.29+17 000/4.58Tin units of seconds·moles/liter. Induction periods are about eight times longer. The following reaction mechanism is most consistent with the data: H+O2=OH+Ok1=1/(4τionization[O2]),O+C2H2=OH+C2Hk2,OH+C2H2=H2O+C2Hk3,C2H+O2=2CO+Hk4,C2H+O2=CO2+CHk5. Reaction (1) is rate-controlling, the above-defined value of k1 agreeing well with the work of others. The time constants τ for CO, CO2, and H2O are identical, indicating that they are all formed in the branching chain reaction, but those of ionization and luminescence are half as large. Diacetylene formation is observed in acetylene-rich mixtures. Experiments with added CO indicate that Reaction (5) is initially the main source of CO2. The reactionC2H+O=CH(A2Δ)+CO  k6explains the observed chemiluminescence well. Experiments with the mass spectrometer show that C3H3+ is the first ion observed. Other experiments suggest that the simple reaction CH(A2Δ)+C2H2=C3H3++e— k7 is not responsible for the ionization. Other possible mechanisms for the formation of C3H3+ are discussed.