C2H5OH2+, C2H5OHD+, and C2D5OHD+ have been studied using high-energy collision-induced decomposition (CID) mass spectrometry. The spectra, in particular the isotope distributions in the CID product ions, vary significantly depending on the reaction conditions of pressure and temperature under which the protonated ethanol or its deuterated equivalents are created. C2H5OHD+ and C2D5OHD+ were formed specifically by the deuteron or proton exchange reactions : (C6D5F)H+/C2H5OH and (C6H5F)D+/C2D5OD, respectively. The reaction exothermicity is too small to allow the formation of the isomeric deuterated ethylene-water complex (=C2H4. H3O+) or isotope scrambling in the ion source. The variations in the CID spectra, dependent on the precursor ion temperature, are therefore highly unusual and unexpected. They indicate that high-energy CID in these molecules is sensitive to the degree of internal rotation of the -OHD around C-O and -CH3 around the C-C bond. This explains variations observed previously for CID spectra of (C2H5OH)H+ which have been attributed to the independent existence of two isomers formed by protonation of ethanol. The work supports the conclusion of Fairley et al. that the H3O+/C2H4 reaction leads to the formation of the lowest-energy isomer of protonated ethanol. It also gives a fresh insight into the mechanism for the collision-induced decomposition of C2H5OH2+.