pi-A isotherms and Brewster angle microscopy (BAM) images of 4-octadecanoxycinnamic acid (OCA) monolayers spread from different concentrations of the solutions indicated that the OCA monolayers depended on the concentrations. The dilution of the spreading solutions could have effectively weakened the intermolecular aggregation in the OCA monolayers. The OCA Langmuir-Blodgett (LB) films were successfully deposited on hydrophilic substrates by selecting a suitable concentration of 1.0 x 10(-4) mol dm(-3) at 18 +/- 1 degrees C. The LR and UV spectra indicated that the OCA molecules in LB films can dimerize under UV light irradiation with a second-order process by use of spectrophotometry. The second-order rate constant k obtained from the absorbance changes at 257 nm is 2.98 x 10(-2) mol(-1) dm(3) s(-1). X-ray diffraction patterns of the OCA LB films are almost the same before and after irradiation, suggesting that the photodimerization only occurred in each monomolecular layer and did not change the periodic structure of the OCA LB films. The syn head-to-head photoproduct is very characteristic and definite due to the ordered reaction environment provided by the OCA LB films. A possible reaction mechanism has been proposed.