The electron spin resonance absorption and electrical resistivity have been measured under rigorous conditions for a series of polypyrroles prepared over the temperature range 120-500�. When plotted as a function of pyrolysis temperature the resistivity shows a maximum in the region 200-300�. Although the resistivity of the polymers prepared at 120� and 500� is roughly similar, their e.s.r. behaviour is quite different. The low-temperature polymer, containing much complexed iodine, shows a very broad signal arising from an excited state probably associated with a polypyrrole-iodine charge-transfer complex; the high temperature polymer, containing no iodine, shows a strong narrow signal arising from the ground state of the polymer. The changes in conductivity of the polymers following the adsorption of electron acceptor or donor molecules have been measured. It is concluded that, depending on the relative amounts of electron donating or attracting chemisorbed species in relation to the concentration of donor nitrogen atoms in the polypyrrole, the polymer may behave as an intrinsic or extrinsic semiconductor with n- or p-type characteristics. Charge-transfer complexes of strength sufficient to cause partial ionization induce extrinsic behaviour by changing the ratio of the number of electrons to the number of holes. Substituent groups such as the hetero atoms which interact with the π-electron system inductively or through resonance affect only the relative mobility of the charge carriers and induce intrinsic behaviour.