The photochemistry of acetylene adsorbed on NaCl(100) was modeled using the unrestricted Hartree-Fock method for the ground state and the CI-single method for the electronic excited states. After comparison of the results from the CI-single method with those of previous work, a number of reactive processes were studied, including the cycloaddition of two adsorbed molecules, one of which is situated in the first excited tripler state, the dissociation of the tripler state adsorbed on the perfect surface, and the dissociation of C2H2 adsorbed on a surface defect in ground or triplet state. The intermediate stationary states of the photolysis have been identified. The results are compared with experimental results.