A combined spectroelectrochemical study by ESR/UV-vis-NIR as well as FTIR spectroscopy on the influence of the copolymer composition on the stabilization of charges upon electrochemical p-doping is presented. As compared to the parent homopolymers 3-hexylthiophene (3-HeTh) and 3-methylthiophene (3-MeTh) which seems to be irregular, FTIR studies of the copolymer of both monomers (copMeHeTh) point to a regioregular structure. The in situ ESR and UV-vis-NIR spectroelectrochemistry at higher doping levels of the polymeric materials proves bipolarons and polaron pairs as stable charged states in poly(3-hexylthiophene) as well as the copolymer copMeHeTh. During the p-doping of poly(3-methylthiophene) bipolarons are the dominating species at higher doping levels. It is demonstrated that only the simultaneous use of both the ESR and the UV-vis-NIR spectroscopy enables the differentiation of polarons (paramagnetic) and polaron pairs (diamagnetic) in a conducting polymer.