The molecular area change of 10,12-pentacosadiynoic acid (PDA) monolayer was recorded and large molecular area expansion was observed during UV-polymerization. The expansion speed experienced a quick increase to maximum and then a steady decrease to zero and finally the area change was converted into a slow contraction. The rate of molecular area change depended on the reaction temperature. Turning off the UV-light during the experiments resulted in area contraction. The contraction speed was described with a kinetic equation, which suggested an accompanied area relaxation during the polymerization. A model for the reaction kinetics (both based on the polymerization and the area relaxation) was proposed and well corresponded to the experimental data. By using this model, the 'activation area' was calculated to be 0.127 nm(2) which was little dependence of the reaction temperature. The positive activation area during the monolayer polymerization indicated that there was an increase in area when the activated complex was formed. The rate constant decreased upon elevated surface pressure. The possible effect of visible-light on the monolayer UV-polymerization was investigated in detail by periodically switching on and off the visible light while keeping the UV-light on. An acceleration phenomenon of the visible light on the monolayer UV-polymerization was observed. The acceleration effect was illustrated by the polymerization mechanism and the electronic state of the polymerization intermediates. (C) 2003 Elsevier Science Ltd. All rights reserved.