Electrochemical effects on self-assembled monolayers of alkanethiols, CnH2n divided by 1SH (n = 9, 10, 18), on Au, Ag(111), and Pt(111) are investigated using sum-frequency generation. Layers on Au and Ag(111) show no noticeable effects. Layers on Pt(111) show gauche transformations that are reversibly eliminated at negative or positive potentials.