For exposure of CO and of ${\mathrm{O}}_{2}$ we studied adsorbate-induced changes in the broadband infrared transmission of ultrathin continuous films of iron and of copper at normal incidence of light. The observed broadband changes correspond to additional conduction electron scattering, but also to charge transfer by the adsorbate, since both of these effects alter the dynamic conductivity of the films. We succeeded in calculating both the amount of charge transfer and the additional relaxation rate from infrared transmission spectra. However, proper dynamic-conductivity data of the pure metal films were crucial for this spectral analysis.