A representative sample of the vegetation of semiarid south-eastern Australia was analysed by catenation, i.e, a multivariate method for detecting and defining compositional gradients or coenoclines. Both floristic and dominance data yielded two catenae. The major one (A) gave the following sequence: heath, temperate mallee, semiarid mallee, belah and pine woodland, shrub woodland, pearl bluebush, bluebush, saltbush. The minor catena (B) differentiated eastern and western aspects of the woodland and shrub woodland. Stepwise multiple regression on environmental variables showed that most of the variation in catena A could be accounted for by a variable expressing the ratio of rainfall to soil water capacity (= depth of wetting by prevalent rainfall). Further contributors to the regression were the proportion of exchangeable calcium, the salinity, and the available phosphate. This leads to the hypothesis that the composition of vegetation in the area is determined mainly by the distribution of soil moisture between shallow and deep layers, and between easily available and marginal (below - 15 bars) moisture. Both are affected by texture and salinity, and different groups of species have adapted to each distribution. Catena B is related mainly to summer rainfall and to exchangeable calcium.