Methyl chloride and CH 3ClCD 3Cl, CH 3ClBr 2 and CD 3ClHI mixtures have been photolyzed at 163.3, 147.0 and 123.6 nm in the gas phase and methyl chloride has also been photolyzed at 147.0 nm in the liquid phase. Over this range of wavelengths, there are four principal primary processes: ▪ At 163.3 nm, process (1) is almost exclusively responsible for the photodecomposition. However, at shorter wavelengths processes (2), (3) and (4) increase while process (1) decreases in importance. At all wavelengths these four processes account for a total quantum yield of 0.75 – 1.0.