Thiophene and its disubstituted derivatives, such as 3,4-ethylenedioxythiophene (EDOT), 3,4-dimethoxythiophene (DMT), 3,4-propylenedioxythiophene (PDOT), and 3,4-butylenedioxythiophene (BuDOT) were oxidized in organic solvents and in ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM(+)PF₆⁻) at RT and under cryogenic conditions. Their radical cations were spectrally characterized at 77 K. Annealing of the irradiated matrix, which triggers the diffusion processes, led to spontaneous oligomerization. The oxidative coupling between a radical cation and a neutral monomer was identified as the first step of the oligomerization process. The scale of oligomerization could be extended by the addition of chloroform, which acts as a dissociative electron scavenger, whereas the dichloromethylperoxyl radicals formed in the reaction with the dissolved oxygen act as secondary oxidizing agents.