Protonolysis of diphenylmethane (1) and three oligophenyl alkanes (2 to 4) has been studied by chemical ionization mass spectrometry of some deuterium-labelled analogues. 1H+ ions reveal that intra-ring proton equilibration is complete within 10 [mu]s whereas inter-ring proton exchange is slow. 2H+, 3H+, and 4H+ ions, additionally, undergo complete inter-ring proton equilibration involving all of the 16 or 21 protons at the aromatic rings. A surprising double C6H6 elimination is found to occur in 2H+, 3H+, and 4H+ ions, suggesting that C6H6 acts as a ‘solvent’ of [MH - C6H6]+ and [MH - 2C6H6]+ ions prior to final fragmentation.