Resonance Raman, ESR, and UV-vis-NIR techniques were used to characterize the secondary doping process in polyaniline (PANI). Experimental results obtained with the first two techniques showed the transformation of diamagnetic quinoid segments into radical semiquinone units while UV-vis-NIR experiments showed the increase of free carriers concentration due to extended conformation. Thermal effects on primarily and secondarily doped polyaniline were also studied by TGA, Raman, ESR, and UV-vis-NIR techniques. The appearance of new Raman bands with temperature was assigned to the formation of "phenazine-like" structures provoked by cross-linking. ESR and UV-vis-NIR experiments showed the absence of,formation of addition radicalar structures due to thermal treatment in contrast to that observed after treatment with m-cresol.