We describe an experimental setup suitable for the measurement of photomodulated IR reflection/absorption spectra of monolayers on a flat gold substrate at temperatures from room to 14 K. Results obtained for a monolayer of C18D37SH on gold are shown and compared with IR spectra of C18D37SH in a KBr pellet at these temperatures. Band widths are reduced by only ∼10%. Upon cooling below 100-150 K, the frequencies and intensities of vas(CD3) and vas(CD2) bands in the C-D stretching region in both types of spectra undergo reversible changes attributed to disappearance of gauche defects by analogy to the behavior of previously studied chains.