A comparative study on polyfluorene (PFl) growth has been carried out in dichloromethane (CH2Cl2) and acetonitrile (CH3CN) solutions employing cyclic voltammetry, kinetic measurements, electrochemical quartz crystal microbalance (EQCM) and in situ spectroelectrochemical techniques. The results indicate that fluorene is polymerized more effectively in CH3CN than in CH2Cl2, and the PFl films prepared in CH3CN generally have better physicochemical characteristics than those in CH2Cl2. The polymerization reaction rates fluctuate due to the passivation/depassivation processes taking place in CH2Cl2, while they decrease steadily with increasing potential cycle number in CH3CN. EQCM studies show that the current efficiencies are about 100% in CH3CN, while they average about 25% in CH2Cl2. Spectroelectrochemical studies show that monomers are quickly polymerized to make long-chain oligomers in CH3CN, while the chain lengths are generally shorter for polymers prepared in CH2Cl2. These differences are attributed to the differences in solvent characteristics such as their dielectric constants, leading to a different solubility of oligomers and different ion stabilization. (C) 2004 The Electrochemical Society.