Trimerisation of E- and Z-alpha-phenylcinnamic acid was investigated by semiempirical quantum chemical methods (AM1 and PM3). Full geometry optimisation and the calculation of hydrogen bond parameters (bond length, bond energies) were performed for cyclic and catemer trimers. Formation of cyclic trimer proved to be more favoured for the E isomer. while the proportion of the catemer and cyclic forms was comparable in the case of the Z isomer. LR spectra of both isomers were simulated and found to show the spectral features of the measured ones.