The electrochemical growth of polyalkylterthiophenes has been monitored in situ with the Electrochemical Quartz Crystal Microbalance (EQCM). Potential controlled synthesis of poly-3＇4＇-didodecyilterthiophene (3＇4＇-DDTT) and poly-33 "-DDTT revealed an unexpected cathodic precipitation following the anodic deposition. From the analysis of the experimental data we obtained an excellent fit of the cathodic precipitation curves with a first-order nucleation law. A correlation between substitution pattern of the starting monomers and the deposition kinetic of the resulting polymers was found. Factors controlling the cathodic deposition of the Poly-DDTTs didn＇t depend on the presence of a parent phase on the electrode. Finally, long time-scale experiments (500 s) with EQCM during potentiostatic synthesis of Poly-DDTTs revealed an evolution of the deposited mass with peculiar features of a process under diffusion control.