The mean kinetic energy of molecular ion fragments derived from the variance of their time-of-flight distribution is found to be in agreement with the values based on their momentum spectra, measured simultaneously. Electron impact on CO2 is taken as a test case. The problem of estimation of mean kinetic energy of ions of a given species arising from differing precursor states is also addressed. This analysis verifies the proposition, that the variance of the time-of-flight distribution is a robust estimate of the mean kinetic energy, as opposed to the commonly used estimate based on the square of the FWHM, which is an under-estimate. We also suggest a reliable method for determining the baseline and the extent of the time-of-flight peaks. © 2006 Elsevier B.V. All rights reserved.