X-ray diffraction is an ideal technique for the in situ study of single crystal metal surfaces in an electrochemical environment. In this paper, measurements of the low-index surfaces of An and Pt are described, in particular with reference to surface expansion effects. Surface expansion can be probed potentiodynamically to correlate expansion with the adsorption/desorption of solution species. In general, the results are in good agreement with recent theoretical calculations. The X-ray technique can also give insight into electrocatalytic reactions as shown by the results for the adsorption and oxidation of CO on Pt(111).