Mixtures of C2H4 with 02 or N3O and argon have been ignited with reflected shocks. The temperature dependence of the induction period to explosion suggests that the rate-controlling reaction for the C2H4-02 system is CH2O + O2 + CHO +HO2. Results for the C2H4-N2O system do not identify one particular reaction as being rate-controlling. A combination of reactions is probably involved. Possibilities include the pyrolysis of ethylene and of ethylene oxide and the reaction CH2+N2O → CH+OH+N2.