The atomic and molecular hydrogen elimination processes from ethylene have been studied using a molecular beam apparatus. site and isotope effects on the molecular hydrogen elimination from ethylene have been clearly observed from the photodissociation of ethylene at 157 nm. Experimental results show that there are three different types of molecular elimination processes: 1,1 elimination , 1,2, cis elimination and 1,2 elumination. Significantly differences have been detected between 1,1 elimination and 1,2 eliminations in their kinetic energy distributions. Noticeable difference elimination. Branching ratios for atomic and molecular hydrogen elimination processes have also been determined for ethylene and its isotopomers. Isotope and site for effects on the branching ratios of differenct molecular elimination channels have been observed. The experimental results are also compared with recent theoretical studies. (C) American Institute of physics. [S0021]-9606(00)01345-3.