Pulsed laser‐induced desorption is used to study the initial C–H bond cleavage of ethylene on Ni(100) in real time. The dependence of the rate of bond breaking on surface coverage, surface temperature, and isotopic substitution is measured. For coverages below 0.4 monolayer, dehydrogenation of C2H4 is unimolecular with kinetic parameters: log ν=9.8±0.5 s− 1 and E a =8.4±0.4 kcal mol− 1. At higher coverages the reaction rate is slower. There is a pronounced kinetic isotope effect (KIE). The rate of C2H4 dehydrogenation is roughly 60 times the rate of C2D4 dehydrogenation at 170 K. A secondary KIE is also observed for 1,1‐ and 1,2‐C2D2H2. Although the bond broken in both cases is a C–H bond, the rate for C2D2H2 is 1/3 that for C2H4. The magnitudes of the KIE’s indicate that a C–H stretching vibration is probably the dominant mode involved in the bond breaking but that other modes contribute significantly.