We report experimental studies of the statics and dynamics of diffusion-controlled polymerization. The polymerization involves the random aggregation of a neutral precursor, pyrrole monomer, in a two-dimensional electrochemical cell under a variety of conditions. As the oxidation potential is increased, the fractal dimension drops sharply from values near 2 (compact structures) to 1 as the growth becomes dendritic. At higher potentials, where the polymerization becomes diffusion limited, a continuum of structures is observed as the dendrites become more irregular and the fractal dimension increases asymptotically to 1.74 0.01. The mean branch angle is found to decrease sharply from 90°in the dendritic regime to approximately 45°in the diffusion-limited regime. In this regime the width of the active growth zone increases at the same rate as the fractal radius. The spectral dimensionality determined by scale-dependent conductivity measurements is 1.26 0.04.. AE © 1987 The American Physical Society.