The gelation rates of poly(3-alkyl thiophenes) (MAT) are strongly dependent on the pendent alkyl chain length. Poly(3-hexyl thiophene (P3HT)) gels at higher isothermal temperatures than that of poly(3-dodecyl thiophene), (P3DDT). Gelation rate has been expressed as a combination of a temperature dependent function f(T) and a concentration dependent function (phi(n)); phi being the reduced overlapping concentration of MAT. From the slope of double logarithmic plot of gelation rate and phi, n values are calculated to be 0.52 and 0.60 for P3HT and P3DDT, respectively. These values are close to the value of percolation exponent beta (0.45) for three-dimensional lattice; so both the gels may approximately obey three dimensional percolation model. The f(T) of gelation rate consists of two steps (i) coil-to-rod transformation followed by (ii) fibrillar crystallization. The coil-to-rod transition is accompanied by a red shift in the pi-pi* transition band in UV-vis spectra. Flory and Weaver theory is used to understand the coil-to-rod transformation process. The theory of fibrillar crystallization extended to dilute solution has been employed to understand the crystallization process. The results clearly indicate that the activation energy of conformational change (Delta F) is higher for P3DDT than that of P3HT. But the free energy change for the formation of critical size nucleus (Delta G(dil)*) is lower for P3DDT than that of P3HT. A comparison of Delta F and Delta G(dil)* values indicate conformational ordering is the rate-determining step for MAT with dodecyl substituent whereas crystallization is the rate-determining step for MAT with hexyl substituent. (c) 2006 Wiley Periodicals, Inc.