Sum-frequency vibrational spectra of monolayers of octadecanethiol on gold and silver have been obtained with a tunable mid-infrared laser beam (3.3-3.6 mu m) and a pump beam at two different wavelengths : 1064 and 532 nm. With pump radiation at 1064 nm the non-resonant background from silver is very weak but the non-resonant background from gold is still comparable with the resonant signal. The advantages and disadvantages of the two pump wavelengths in the study of adsorbates an silver and gold electrodes are discussed.