The Mont Saint-Hilaire alkaline complex, Québec, is a Cretaceous rift-related intrusion comprising two gabbroic suites and the East Hill suite, an assemblage of several distinct nepheline syenites and evolved syenitic rocks. Whole-rock analysis of the East Hill suite reveals two fractionation trends, one which is described by the syenitic lithologies and one by the evolved syenites and syenoids. A lamprophyric unit stands on its own, geochemically, consistent with typical lamprophyre petrogenesis. Whole-rock geochemistry of the East Hill suite shows moderate silica content but highly alkaline rocks, enriched in Mn, Zn, Zr, Y, and Ce. Chlorine and sulfur are important and present in abundant secondary sodalite and pervasive microsulfides. Mineral geochemistry indicates decreasing oxygen fugacity with cooling, and phases such as titanite (present) and aenigmatite (absent) defined boundaries in fO2 – T space. Oxygen fugacity data combined with petrography and geothermometry defined an 2 fO – T curve for the East Hill suite magma, revealing that the oxygen fugacity ranged between approximately 10-10 bar at intrusion down to 10-25 bar subsolidus, and that, owing to non-linearity of oxygen buffers, the 2 fO – T path crossed below QFM early, went above QFM late in the crystallization history and then above HM with cooling. A similar procedure applied to sulfur fugacity generated a trend of initial increase from about 10-4 to 10-2 bar followed by decreasing sulfur fugacity with cooling down to approximately 10-8 bar. The oxygen trend implies that earlier suggestions that oxidation of iron drove the evolution of pyroxenes towards aegirine are incorrect. Textural evidence of abundant alteration and secondary mineralization suggests subsolidus action of alkaline fluids. Contrary to previous studies, most sodalite in the East Hill suite is not primary but is the product of reaction of nepheline with chloride-rich fluids after crystallization. Sodalite pseudomorphs continued to react with these fluids, whose chemistry changed with time to silica- then carbonate-rich, resulting in further reactions that led to natrolite and dawsonite deposition, accompanied by late-stage rhodochrosite. It is hypothesized that the silica- and carbonate-rich phase of these fluids caused the decomposition of a melilite groundmass in the lamprophyre to a natrolite-calcite assemblage.