Methods of measuring initial energies of ionized molecular fragments are discussed in terms of the velocity space. An ion source together with the collector arrangement define a certain part of the velocity space from where ions are collected. If this part is not equivalent to a fixed solid angle in velocity space, discrimination due to initial energy takes place. Well defined discrimination effects can be used to measure the initial energy probability function WE of an ion ensemble. Emphasis is laid especially upon the deflection method. Using this method one obtaines the probability function WE simply by differentiating the primarily measured ion current distribution. The necessary working conditions which have to be fulfilled are discussed in some detail and an experimental arrangement is described, which has been used to measure the initial energy distribution functions of fragment ions from the lower paraffins.