Poly(N-vinylcarbazole) powder (PVK) has been doped with bromine at 293, 303, 323, 363, 383 and 423 K for 24 h. These powders have been systematically studied by electron spin resonance (ESR) at temperatures varying from 300 to 523 K and reverse. It is shown that the stability of the ESR signal after “temperature cycling measurements” depends on the doping temperature. The modifications of the signals during and after measurements are explained by bromine reaction with PVK. The PVK is partly decomposed with formation of carbazole monomers embedded in a disordered polymer matrix. The degree of degradation of PVK depends on the doping temperature. It has been checked by X-ray diffraction and X-ray photoelectron spectroscopy (XPS). Moreover a correlation between room temperature conductivity and PVK modification is established.