Decomposition of 2-chloroethylphosphonic acid (Ethephon) was studied in dried films at various water vapor pressures (0.6 to 86.9 millimeters Hg, 3.2 to 93.9% relative humidity) and temperatures (20, 30, 40, and 50 C) at pH 6.3 and 7.0. The rate of decomposition could be determined equally well by [(14)C]Ethephon or ethylene measurements. The rate increases at increasing water vapor pressures at a constant temperature and pH, up to an optimum. The optimum vapor pressure for decomposition approximately doubles for each 10 C increase. The activation energy for the decomposition reaction in water vapor pressures of 3.2 to 12 millimeters Hg is 8.7 and 14.3 kilocalories per mole at pH 6.3 and 7.0, respectively.Decomposition of Ethephon is inhibited above an optimum vapor pressure. The inhibition is stronger at lower temperatures and at pH 6.3 than at pH 7.0. The rate of decomposition and the inhibition observed at a low temperature (20 C) was found to be similar on various surfaces, including olive leaves.Failure to induce olive (Olea europaea L.) fruit abscission under certain environmental conditions can be readily attributed to rapid breakdown of Ethephon at elevated temperatures and low relative humidities.