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.