The Île Cadieux alnöite, a small Cretaceous intrusion, is a member of the Monteregian Hills Petrographic Province. Mineral chemistry was determined for olivine, spinel, clinopyroxene, melilite, and a potassium–iron–nickel sulphide akin to djerfisherite. Whole rocks were analysed for major elements and 24 trace elements (including rare-earth elements). The Île Cadieux alnöitic rocks have greater MgO/Al2O3 ratios than most alnöites but are otherwise similar to typical alnöites with respect to major and trace elements. Comparison of spinel and whole-rock chemical compositions from the Île Cadieux and Île Bizard (located 15 km to the northeast) alnöitic intrusions suggests that these magmas were generated under different physicochemical conditions of melting, or alternatively that they are fractions of a single magma which crystallized under different conditions.