The technique of atomic resonance absorption spectrophotometry (ARAS) was used to study the generation of H atoms during the thermal decomposition of C 2 H 4 . Highly diluted mixtures with relative concentrations between 20 and 800 ppm of C 2 H 4 in Ar were used. The experiments were performed behind reflected shock waves at temperatures between 1700 and 2200°K and densities between 6·10 18 to 1.6·10 19 particles/cm 3 . For reaction (1) C 2 H 4 + M →C 2 H 3 +H+ M we obtained the rate coefficient expression: k 1 =(3.8±1.3)·10 17 exp(− E 1 / R·T ) cm 3 /mol·sec E I / R =49 400±900°K Together with previous measurements for reaction (1a) C 2 H 4 + M →C 2 H 2 +H 2 + M which gave k 1a =(2.6±0.5)·10 17 ·exp(− E 1a / R·T ) cm 3 /mol·sec E 1a / R =39 900±500°K it became apparent that C 2 H 4 decays simultaneously via two reaction channels under the chosen conditions. In addition it was possible to derive approximate rate coefficients for reaction (3) H+C 2 H 4 →C 2 H 3 +H 2 k 3 ≅1.5·10 13 cm 3 /mol·sec; T =2000°K k 3 ≅5·10 12 cm 3 /mol·sec, T =1700°K