Fall-off curves for the reaction ClO + NO2(+N2)->ClONO2(+N2) at 300 and 220 K have been calcd. from Kassel integrals by use of a theor. derived high-pressure limit and the exptl. detd. low-pressure limit as ref. points. The reaction deviates considerably from the low-pressure limit at pressures >10 torr. Effective rate consts. for use in atm. models of ClONO2 formation are derived.