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A taxonomic revision of the highly diverse Western Australian Mulga flora (Acacia aneura F.Muell. ex Benth. and its close relatives) is presented, based on morphological analyses of almost 2000 herbarium collections complemented by field studies of about 300 populations (mostly located in Western Australia, with a few from the Northern Territory and South Australia). Twelve species accommodated in three informal groups, the Blue, Grey-green and Green Alliances, are recognised. Almost all of the species contain informal variants, and putative hybrids and/or intergrades are common. Because many of the species had previously been recognised as varieties of A. aneura (Pedley 2001) a discussion of our taxon concepts is given. The 12 species are defined by a combination of morphological characters, the most important being branchlet resin (translucent vs opaque) and pod margins (rimmed, bevel-edged or winged), complemented by new shoot (resinous vs non-resinous), phyllode (shape, size, curvature, nervature) and pod (width) attributes. A discussion of the taxonomically most informative characters in the Mulga group is presented. Seven new species are recognised: Acacia aptaneura Maslin & J.E.Reid (syn. A. aneura var. pilbarana Pedley and A. aneura var. tenuis Pedley), A. caesaneura Maslin & J.E.Reid (syn. A. aneura var. argentea Pedley), A. fuscaneura Maslin & J.E.Reid (syn. A. aneura var. fuliginea Pedley), A. incurvaneura Maslin & J.E.Reid (syn. A. aneura var. microcarpa Pedley), A. macraneura Maslin & J.E.Reid (syn. A. aneura var. macrocarpa Randell), A.  mulganeura Maslin & J.E.Reid and A. pteraneura Maslin & J.E.Reid. Acacia aneura var. intermedia Pedley is provisionally regarded as conspecific with the broadly circumscribed A. aneura.

Acacia aneura F.Muell. ex Benth. is a member of a large, taxonomically complex group of plants that are very common in the Australian arid zone. In order to help determine the application of this name the type collection of A. aneura at the National Herbarium of Victoria (MEL) is reassessed after visits to the type locality. This collection comprises rather fragmentary specimens mounted on three sheets (MEL 724215, 724218 and 724219), each labelled as having been collected [in 1851] by Ferdinand von Mueller from ‘Cudnaka’. This locality is now known as Kanyaka, located in the South Flinders Ranges, South Australia, between Quorn and Hawker. The holotype is confirmed as the depauperate fruiting specimen on MEL 724218. The fertile specimen on MEL 724219 is confirmed as belonging to a Mulga taxon of uncertain status and is not a type. The sterile specimen on MEL 724215 and the sterile specimens on MEL 724219 may or may not be types, but their status cannot be determined with any certainty. The populations at Kanyaka that Mueller presumably visited were sampled by the authors in 2007, 2008 and 2010. Subsequent study showed there to be two distinct Mulga morphotypes in these populations. One morphotype corresponds to the type of A. aneura but the status of the second morphotype is uncertain. The latter does not match the fertile specimen on MEL 724219 and appears not to have been collected by Mueller; it may possibly represent a hitherto undescribed species. The taxonomic status of the first morphotype, and hence the application of the name A. aneura, is currently uncertain. Further field, morphological and genetic studies, especially of South Australian populations, are needed to resolve the application of the name A. aneura and also to establish the status of the second morphotype collected from the type locality.