The effective charge separation has been verified by two light induced ESR (LESR) signals which were observed under light excitation. These signals can be attributed to positive polarons (P⁺) on the polymer chain and to radical anions of PCBM (P^-). In order to study the mechanism of photoconductivity the careful analysis of ESR and LESR linewidth broadening in donor/acceptor (P3DDT/PCBM) composit thin solid films have been carried out by X- and W-bands operated ESR spectrometers. Multifrequency ESR spectroscopic dates give the possibility to separate several parts of additional line broadening mechanisms to extract the dependent of polaron diffusion. The linewidth as a function of ESR frequency is examined by the standard least-square method with several spectroscopic independent parameters. The effective T₂ relaxation time was obtained without the part of the anisotropic broadening of ESR linewidth and T₁ one using the well observed in experiments the ESR line saturation effect. The difference (several times) between T₁ values for P⁺ and P^- radicals is in agreement with results published for analogical composites.