The ¹H n.m.r. spectra of protonated fluorenone, cyclopentadithiophenones (FSO₃H–SbF₅–SO₂ at –60°), and cyclopentadithiophens (CF₃CO₂H at –15°) are discussed. Di- and mono (carbonyl)-protonation occurred. Cyclopenta[1, 2-c : 3, 4-c′]dithiophen-7-one and fluorenone are found to be protonated first at the carbonyl group. PPP Calculations show that carbonyl protonated ketones having a cyclopentadiene carbocyclic ring are energetically less favourable than those having a cyclopentane or cyclopentene carbocyclic ring. From the values of the chemical shift of the OH protons it is concluded that the C–O bond remains largely a double bond. Three cyclopentadithiophens are easily protonated in CF₃CO₂H at –15° at corresponding positions in the b-annulated thiophen ring. The absorption frequencies in the electronic absorption spectra of these protonated cyclopentadithiophens are in good agreement with transition energies calculated by PPP methods.