Thin coatings of photoresponsive, pyrimidine-terminated molecules, attached to gold or quartz substrates in contact with water, undergo dimerization and wettability changes when irradiated with UV light at 280 and 240 nm. Self-assembled monolayers of long chain thymine-terminated thiols give the largest, reversible photoinduced contact angle changes. The latter are caused by a decrease in surface charge as the thymine monomer dimerizes upon irradiation, a process which is accompanied by an increase in the acidity constant of the dimer. Uracil self-assembled monolayers photodimerize but do not photocleave; there is an irreversible change in contact angle. Spin-cast films of thymines give smaller contact angle changes, the maximum values corresponding to films which are composed of a mixture of crystalline and amorphous states.