Isotherms of a PDA monolayer were measured on a subphase of 4 x 10(-4) M CdCl2 at different temperatures. UV polymerization of the PDA monolayer was carried out at different temperatures and surface pressures, and expansions or contractions were observed to depend on the applied surface pressure. The polymerization rate constant was found to decrease with increased applied surface pressure, but it increased with temperature. From the temperature dependence of the polymerization of the PDA monolayer, the apparent activation energy of the 1,4-addition polymerization was estimated to be 63.6-89.7 kJ/mol depending on the surface pressure at which UV polymerization was carried out. The apparent activation energy had a minimum of 63.6 kJ/mol at the surface pressure of 20 mN/m, and there would be an obvious increase of the activation energy when the surface pressure deviated from this value. The surface pressure dependence of the apparent activation energy was interpreted in terms of molecular packing in the monolayer.