We studied the surface femtochemistry of CO/O2/Pt(111) induced with 0.3-ps laser pulses over a wide range of wavelength and fluence. Below 10 J/mm2, the yields depend linearly on fluence. Above 10 J/mm2, the yields scale nonlinearly in the fluence. From the dependence of the yields on wavelength, we determine that the nonlinear surface femtochemistry is influenced by nonthermal substrate electrons.