Two types of paramagnetic centers in slightly and medium doped polyaniline were detected by 3cm and 2-mm waveband EPR measurements. Spin–lattice and spin–spin relaxation times, as well as frequencies of the intra- and interchain motion of mobile centers were determined separately by a steady-state saturation technique in the temperature range 90–330 K. The charge transport in slightly doped polyaniline can be described within the framework of the Kivelson theory, while the process in moderately doped polymer proceeds predominantly by the mechanism of variable range hopping. It is shown that doping of the polymer leads to the formation of small polarons, which are the major paramagnetic charge carriers in metal-like polyaniline, and bipolarons. In the ultimately doped poly(aniline), the concentration of diamagnetic bipolarons may exceed the concentration of polarons