The pi-A isotherms of a 10,12-pentacosadiynoic acid (PDA) monolayer were measured on a subphase of 2 x 10(-4) M CdCl2 at varying pH and temperatures. UV polymerization of PDA monolayers was carried out at different pH values, surface pressures, and temperatures. The dynamic elasticity of PDA monolayer has been measured both before and after polymerization by means of a dynamic oscillation method, i.e., measuring the changes of surface pressure caused by an oscillating barrier at given frequency and amplitude. The dynamic elasticity of PDA and its polymer PPDA monolayer was found to decrease with increasing of temperature but to increase with increasing of pH. The dynamic elasticity of PDA monolayer increased with increasing of surface pressure but it had a sudden decrease at 25 mN/m for PPDA monolayer. The dynamic elasticity of PPDA monolayer was even smaller than that of its monomer PDA monolayer and decreased with increasing of polymerization time. The explanation based on conformation change has been presented. It was interpreted in terms of molecular packing in the monolayer.