Photoresponsive surfaces were prepared by attaching pyrimidine-terminated molecules to flat gold Substrates (as thiol self-assembled monolayers) or by dip-coating quartz surfaces. Both types of films underwent photodimerization (two pyrimidine rings react with one another and form a cyclobutane type dimer through the C5=C6 double bond) when irradiated with light of 280 nm wavelength. The reverse reaction was carried out by irradiating the dimerized surface with light of 240 nm wavelength. The photoinduced chemical changes are accompanied by a change in the physical properties of the surface (e.g., wettability and acidity), and are highly dependent on the structure of the photoactive molecules. The surface dimerization reaction follows a pseudo-first order reaction. The rate constant is determined by the structure of the pyrimidine headgroup, In self-assembled monolayers, uracil derivatives dimerize faster than thymine derivatives due to a reduced steric repulsion near the reaction center. fit dip-coated Films, however, uracil derivatives appear to be less ordered and, correspondingly, the efficiency of the reaction is lower. The reaction rate is also very sensitive to the ordering within the layer, which can be manipulated through the structure of the tail. In SAMs, faster dimerization occurs with molecules containing flexible chains. In dip-coated Films, the presence ora polar group at the chain terminus favors dimerization.