A convenient kinetic uptake method has been employed to determine the absolute saturation monolayer coverage of C2H2and C2H4on a Si(100)(2×1) surface. Such measurements are important for postulating the structure of the chemisorbed hydrocarbon species on this surface. The saturation surface coverage for both chemisorbed molecules at 105 K is 2.5(±0.2)×1014molecules/cm2. This number is consistent with 1 hydrocarbon molecule per Si dimer site at monolayer coverage when the role of surface defects on Si(100) is considered. A di‐&sgr; bonding model for both molecules is proposed at saturation coverage.