We studied the surface femtochemistry of CO/O-2/Pt(111) induced with 0.3 ps laser pulses over a wide range of wavelength and fluence. Below 10 mu J/mm(2), the yields depend linearly on fluence. Above 10 mu J/mm(2), 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.