The problem of ion structure and energy is discussed briefly and attention directed to the existence of two types of structural information. Studies of ion formation reactions bear on the initial structure of the ion, whereas decomposition reactions relate to the ion at a later time, after structural rearrangement has had chance to occur. The desirability of conducting both types of investigation concurrently is emphasized. Of the available experimental methods at this time, kinetic measurements for formation studies and metastablel ion relative abundances for decomposition studies have been utilized, although other techniques may eventually prove preferable in the future. The [M – Y] reaction of a series of substituted benzyl substrates XC6H4CH2Y, where Y = OC6H5 and H, has been examined, and the present results are compared with previous work on ion structure and energy in this area, with the aim of evaluating the potential of the newer techniques. Copyright © 1969 John Wiley & Sons, Ltd.