Hydrous melting experiments on a basaltic andesite from Late Volcano in the Tonga Arc were conducted at 900–1250 °C and 1 atm to 2.5 GPa, with a range of added H2O concentrations (0–9 wt%). These were used to constrain conditions of phenocryst precipitation in the basaltic andesite and to better understand the processes of magmatic differentiation beneath Tongan volcanoes. Comparison between the products of experiments and Tongan lavas indicates that basaltic andesites from Late crystallised plagioclase + augite + pigeonite ± orthopyroxene while water-saturated at comparatively low pressures (≤0.2 GPa) but high temperatures (≥1000 °C). Glasses produced at 950 °C and 0.2 GPa are similar in composition to dacites from neighbouring volcanoes, including Fonualei. This confirms previous interpretations of dacite magma genesis by crystal fractionation of plagioclase and clinopyroxene from basaltic andesite parent magmas, rather than by partial melting of lower crust. This produces decreases in Dy/Yb with increasing SiO2 without a role for amphibole, probably due to the high temperatures and low alkali concentrations involved. Tongan magmas are primarily tholeiitic, and their high water contents bring into question previous hypotheses that attribute the tholeiitic trend to low concentrations of magmatic H2O. Attributes inherited from the Tongan parent magmas, including low alkali concentrations, appear to drive tholeiitic differentiation, regardless of water content.