Mineral and melt inclusions in olivines from the most Mg-rich magma from the southern West Sulawesi Volcanic Province indicate that two distinct melts contributed to its petrogenesis. The contribution that dominates the whole-rock composition comes from a liquid with high CaO (up to 16 wt %) and low Al 2 O 3 contents (CaO/Al 2 O 3 up to 1), in equilibrium with spinel, olivine (Fo 85–91 ; CaO 0·35–0·5 wt %; NiO 0·2–0·30 wt %) and clinopyroxene. The other component is richer in SiO 2 (>50 wt %) and Al 2 O 3 (19–21 wt %), but contains significantly less CaO (<4 wt %); it is in equilibrium with Cr-rich spinel with a low TiO 2 content, olivine with low CaO and high NiO content (Fo 90–94 ; CaO 0·05–0·20 wt %; NiO 0·35–0·5 wt %), and orthopyroxene. Both the high- and low-CaO melts are potassium-rich (>3 wt % K 2 O). The high-CaO melt has a normalized trace element pattern that is typical for subduction-related volcanic rocks, with negative Ta–Nb and Ti anomalies, positive K, Pb and Sr anomalies, and a relatively flat heavy rare earth element (HREE) pattern. The low-CaO melt shows Y and HREE depletion (Gd n /Yb n ≤41), but its trace element pattern resembles that of the whole-rock and high-CaO melt in other respects, suggesting only small distinctions in source areas between the two components. We propose that the depth of melting and the dominance of H 2 O- or CO 2 -bearing fluids were the main controls on generating these contrasting magmas in a syn-collisional environment. The composition of the low-CaO magma does not have any obvious rock equivalent, and it is possible that this type of magma does not easily reach the Earth's surface without the assistance of a water-poor carrier magma.