Résumé : The quantum yield of the gas phase photochlorination of C2Cl4 and C2Cl3H have been measured. The light intensity of a Hg vapour lamp transmitted by a filter has been measured with a potassium ferrioxalate actinometer. The spectral intensity distribution of the light was measured and corrections were applied for intensity loss and multiple reflexions. The quantum yield is given by Φ = Ia −1/2k(Cl2) with log k = –(1065 ± 8)/T + 3.941 ± 0.021 for the photochlorination of C2HCl3. Conditions are described which permit one to use this reaction as an actinometer for other photochlorinations, so as to avoid the corrections mentioned above and permitting one to use the approximation Ia = 2,303. Ioϵl(Cl2) even at high absorption (up to about 180 mm Cl2 with light of about 4300 A). The photochlorination of C2Cl4 has a somewhat more complexe mechanism and is therefore not very adequate for actinometry. Copyright © 1959 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim