A study of the chemical ionisation of two pairs of diastereoisomeric unsaturated tertiary alcohols with isobutane as the protonating gas shows that hydrogen bonding between the protonated hydroxy-group and the double bond can stabilize the quasi-molecular ion, if the geometry is correct. Therefore, it is possible to differentiate between two isomers. With ammonia, the most interesting phenomenon is an SN2-like reaction, in which the OH2+ group is replaced by NH3+, with Walden inversion.