The low-pressure recombination rate consts. of the reactions I + NO + M -> INO + M (with 14 different M) and I + NO2 + M -> INO2 + M (with 26 different M) were measured at 330 K by laser flash photolysis. The collision efficiencies bc are analyzed and compared with other thermal activation systems. Whereas bc increases in 1 reaction with an increasing no. of atoms in M, practically no such effect is found when, for the same M, different reactions with varying complexities of the reacting mols. are considered.