The low-temperature matrix isolation technique in combination with FTIR spectroscopy, has been used to study the trans-cis photoisomerization of p-(dimethylamino)-beta-chlorostyrene (DMACS). The highly dominant form of the compound immediately after deposition in argon matrix has been identified, by comparison of its experimental infrared spectrum with that calculated at the DFT(B3LYP)/6-31G** level, as the trans isomer. Upon UV irradiation of the matrix, a photoreaction was observed resulting in the appearance of a new IR spectrum corresponding to the photoproduct. By comparison with the theoretically predicted spectra, the photoproduct was identified as the cis form of DMACS. Cross-sections of the potential energy surfaces (PES) along the C-1-C-10=C-11-Cl torsional coordinate in the ground (SO) as well as in the lowest excited singlet (SI) states were generated using the semiempirical (AMI) method. Although the relative energies of the two isomers are quite close (4.6 kJ mol(-1), MP2/6-31G** calculation) in the ground state, the PES reflects that attainment of the equilibrium in the distribution of the two isomers is restricted because of a large activation barrier. The simulated PES, however, corroborates a trans --> cis photoisomerization process involving the participation of the S-1 state.