The solvatochromic conformational transformation of electroconductive poly[3-(10-hydroxydecyl)]thiophene in dimethylpropyleneurea solution, due to the addition of poor solvent, was investigated. A red shift of 80 nm in the ultraviolet-visible absorption maximum suggests a much more extended conjugated pi-system for the new conformation. The time scale of this change allows a spectrokinetic analysis as a function of the type and amount of poor solvent. Second-order reaction kinetics are evident in all cases, and this is interpreted within the framework of an intrachain rearrangement leading to a higher coplanarity of thiophene rings in the new phase. A solid phase, laid down at very long times, gives insight about the final steps of an association process.