The crystallization of palladium in poly-3,4-ethylenedioxythiophene (PEDT) films is studied in order to obtain nanosized palladium clusters in PEDT composite layers with high electrocatalytic activity. Nucleation and growth of Pd is investigated for dependences on overpotential, polymer layer thickness d and chemical state (active or overoxidized) of the PEDT films. It is found that before the onset of diffusion limitations the growth occurs under charge transfer control. The observed induction period t(0) indicates crystallization at the metal \ polymer interface. The linear relation I-1/3 versus t found in the initial stage of the deposition process gives evidence for three-dimensional growth of the metal crystals. The number N-0 of sites active for nucleation decreases strongly with increasing d. A saturation in N-0 is reached for continuous thicker films. The overoxidized layers are less active for metal deposition and exhibit a 50-fold lower number of active sites. The electrocatalytic properties of the Pd/PEDT composite layers are studied with respect to hydrogen sorption. High electrocatalytic activity is found for composites obtained with thin PEDT films polymerized in the low potential region.