The petrology of the East Otago Volcanic Province (late Miocene), including the Dunedin Complex, is dominated by undersaturated sodic lavas and shallow intmsives whose compositions range from alkali basalt, basanite, and nephelinite to trachyte and phonolite. A range of relatively potassic volcanic rocks is also developed in the Province. The degree of undersaturation and ratios such as (FeO+Fe2O3):MgO and K2O:Na2O vary widely in rocks with comparable differentiation indices. Chemical and mineralogical data on whole rock-glass pairs and variants of shallow intrusives in the Province, assessed in conjunction with similar data from other volcanic provinces, demonstrate the production of trachytic liquids from alkali basaltic parents and of phonolitic liquids from basanitic parents. Fractionation trends in both sodic and potassic series, defined by variation in differentiation index and normative nepheline, indicate that the degree of undersaturation of the derivative salic liquids is dependent on that of the parent; the slope of the fractionation trend lines is influenced by additional factors, including Po2. The unequivocal production of phonolite from trachyte in the East Otago Province has not been demonstrated. The mafic variants of theralites from Waihola and a basanite-pegmatoid association from Omimi are similar in composition, but the most salic differentiates from these differentiation sequences display pronounced differences in their KO:Na2O ratios, tending towards leuco-theralitic (cf. lugarite) and malignitic compositions respectively. A multicomponent spectrum of fractionation lineages is suggested for individual differentiated bodies and for lava series. Sodic lineages include: (I) alkali basalt–hawaiite–mugearite–benmoreite–trachyte; (2) a more undersaturated series from basanite through nepheline hawaiite, nepheline mugearite, and nepheline benmoreite, to phonolite; (3) a nephelinite series, more restricted in compositional range. The relatively high Fe: Mg ratios characterizing the mugearitic variants are sometimes exhibited by members of the more potassic series in which, however, Fe:Mg ratios tend to be characteristically lower. This feature is correlated with the frequent occurrence of kaersutitic amphibole or its resorption products, indicative of intratelluric fractionation under relatively hydrous conditions and probably relatively high Po2. Certain nepheline trachyandesites from East Otago are described in detail. Trachy-basalt-trachyandesite-tristanite-trachyte and sanidine basanite-nepheline trachyandesite-nepheline tristanite-phonolite lineages are also proposed, and the nomenclature of these more series discussed.