An additively decomposable inequality measure is one which can be expressed as aweighted sum of the inequality values calculated for population subgroups plus the contribution arising from differences between subgroup means. The paper derives the entire class of measures which are additively decomposable under relatively weak restrictions on the form of the index. The subclass of mean indepedent measures turns out to be a single parameter family which includes thesquare of the coefficient of variation and two entropy formulae proposed by Theil.