The unimolecular decomposition of CH 3CH 2OH + 2, protonated ethanol ions, has been studied in a variable-temperature selected-ion-flow-drift tube, and unimolecular rate coefficients, k uni, have been determined as a function of the mean centre-of-mass collision energy of the ions with the helium carrier gas atoms, E c. The only product ions observed are H 3O + and C 2H + 5. Quantitative modelling of the decomposition has been carried out making the following assumptions: (1) the induction period for attaining steady-state distributions is brief compared to the timescale of the experiment; (2) the translational energy distribution of the primary CH 3CH 2OH + 2 ions is Maxwellian with a mean energy E c and that a thermal probability distribution function is appropriate for vibrational energy and angular momentum transfer in collisions between the primary ion and the helium atoms. The experimental results indicate that assumption (1) is valid but that assumption (2) is not, because the values of k uni measured at two different carrier gas temperatures of 300 and 500 K but at the same E c are not the same. In principle, it should be possible to deduce thermal rate coefficients for collisional decomposition at higher temperatures T than are accessible experimentally by extrapolating to zero E-field plots of the experimental rate coefficients against the contribution to E c due to the drift field obtained for fixed values of E c (assuming E c = 1.5 kT). It is shown that the form of the extrapolation greatly influences the value of k uni so derived, and it is apparent that more experimental data and careful modelling are required before meaningful k uni versus T data can be deduced.