A molecular beam study of the reaction of C2H4 with O2 on Ag(111) was attempted and supplemented with data obtained using an isotropic source (IS) configuration. In the beam configuration, the possible product signals at 18 amu (H2O) or 44 amu (C2H4O, CO2) could not be discerned. In the IS configuration, a product signal at 44 amu (CO2) was observed. An estimate of the collision reaction probability of 9×10−3, obtained from the IS data, confirmed that the product signal would not be readily observable in the beam configuration due to signal-to-noise considerations. From C2H4 beam scattering data, coupled with the experimental ion spectrum obtained from C2H4 by electron impact, it was possible to estimate that roughly about 15% of the incident C2H4 fragmented upon collision with the surface; 10% going to C2H3 and 5% to C2H2. From the surface temperature dependence in the IS configuration, an activation energy of ∼8 kcal/mole was obtained for the mass 44 product.