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Abstract: Equilibrium crystallization experiments have been performed on a primitive high-MgO basalt (HMB) from Soufrière, St. Vincent, with three initial H2O contents (0·6, 2·3 and 4·5 wt %), at pressures of 0·4, 0·7, 1·0 and 1·3 GPa and temperatures from 1350 to 950°C. Redox conditions, as determined by µXANES analysis of Fe3+ in experimental glasses, were 1–4 log units above the nickel–nickel oxide (NNO) buffer. The aim of the study was to explore the differentiation conditions that gave rise to the observed geochemical variation in lavas and plutonic (cumulate) xenoliths from St. Vincent. An experiment with 4·5 wt % initial H2O is multiply saturated close to its liquidus (1180°C and 1·3 GPa) with a spinel lherzolite assemblage, which is consistent with a primary origin for HMB in the mantle wedge. Multiple saturation of HMB with 2·3 wt % H2O was not observed, but is inferred to occur at pressures >1·3 GPa. The experimental results show that initial H2O content has significant influence on differentiation paths of primary HMB magma, with different lava varieties generated under discrete, well-constrained P–T–H2O conditions. Low-magnesian basalts (LMB) can be generated from HMB with 2·3–4·5 wt % H2O at pressures of 1·0–1·3 GPa, corresponding to Moho depths beneath St. Vincent. The CaO contents of LMB are sensitive to differentiation pressure: high-CaO LMB are produced at pressures >0·5 GPa. Basaltic andesites (BA) can be generated at 0·7–1·0 GPa from HMB with 0·6–2·3 wt % H2O. High-alumina basalts (HAB) are produced at mid- to upper-crustal conditions (less than or equal to 0·4 GPa) by differentiation of HMB with high initial H2O (less than or equal to 4 wt %) through delay of plagioclase crystallization and dominant fractionation of olivine, clinopyroxene and spinel. St. Vincent andesites could be produced from relatively dry (less than or equal to 0·6 wt % H2O) HMB only at lower-crustal conditions. This is suggestive of a partial melting origin from precursor HMB that had solidified at depth to produce gabbros with approximately 30% hornblende (i.e. approximately 0·6 wt % structurally bound H2O). The experimentally determined differentiation conditions are consistent with polybaric differentiation within a hot zone that extends from the Moho and uppermost mantle to the mid- or upper crust. Within the hot zone differentiation occurs by a combination of crystallization of HMB with 2–5 wt % H2O and partial melting of ancestral HMB gabbros. Although the experimental melts provide an excellent match to erupted lava compositions, experimental crystal compositions do not match either phenocrysts or cumulate crystals, as preserved in xenoliths. The failure to reproduce natural crystal compositions suggests that these are formed as differentiated magmas ascend and attain their H2O-saturated liquid at shallower pressures. Thus there is a disconnect between the high-pressure phase compositions and assemblages that generate liquid compositional diversity and the low-pressure composition and assemblages that occur as phenocrysts and in cumulate xenoliths. This finding lends support to the idea of cryptic fractionation in the generation of arc magmas.