According to the ¹H and ¹³C NMR data, 3-arylpropynoic acids and their esters XC₆H_n-C≡C-CO₂R (R = H, Me, Et) having electron-withdrawing substituents in the benzene ring (X = NO₂, CN, COMe, CO₂Me) exist in HSO₃F at -80 to 0°C as XC₆H_n-C≡C-C⁺(OH)OR ions. Derivatives with other substituents (X = H, F, Me, MeO) in HSO₃F or CF₃SO₃H above -40°C undergo protonation at the acetylenic carbon atom neighboring to the acid group to give unstable vinyl-type XC₆H_n-C⁺=CH-CO₂R cations which are then transformed into mixtures of stereoisomeric (Z and E) fluorosulfonates or triflluoromethanesulfonates XC₆H_n-CY=CH-CO₂R (Y = OSO₂F, OSO₂CF₃), the E isomer prevailing.