Morphological variation within Eucalyptus series Subulatae subseries Decussatae and Decurrentes was assessed by adult and seedling characteristics for 321 individuals representing 91 populations from across the broad geographical distribution of both the subseries. All previously recognised taxa from the subseries are included. Based on examination of adult and seedling morphology and phenetic analyses of morphological variation, eleven taxa (eight species) are recognised within E. subser. Decussatae. Adult morphology is most important in distinguishing taxa in this subseries, although seedling characters are also useful discriminators of some taxa, most notably E. aspersa and E. yalatensis. The previously recognised E. eucentrica is weakly distinguished from E. socialis, and E. socialis s.l. (i.e. including E. eucentrica) forms four adjacent groups in ordination analyses based on adult characters and adult and seedling characters combined. Four subspecies of E. socialis are recognised. Eucalyptus socialis subsp. viridans Nicolle subsp. nov. is described to accommodate green-leaved populations of E. socialis from the Peninsulas and southern Murray Mallee regions in South Australia. Eucalyptus socialis subsp. victoriensis Nicolle subsp. nov. is described to accommodate coarse-leaved, budded and fruited populations of E. socialis from the Great Victoria Desert in Western Australia and South Australia. A new combination is also made [E. socialis subsp. eucentrica (L.A.S. Johnson & K.D. Hill) Nicolle comb. nov.]. Based on the examination of variation in adult and seedling morphology and phenetic analyses of morphological variation, 11 or 12 taxa within E. subser. Decurrentes are recognised. Eucalyptus transcontinentalis subsp. semivestita is considered to be synonymous with E. moderata and the circumscription of this species is greatly modified. A new taxonomy for both the subseries is presented, based on extensive field, seedling and herbarium examination of all taxa in the series and phenetic analyses. Descriptions and keys to the taxa within the subseries are presented.