Mont Saint Hilaire is an approximately 10 km2 alkaline plutonic complex in the Monteregian petrographic province of Quebec. The complex consists of an older, western half of alkali gabbros and an eastern portion of nepheline syenites and magmatic breccias. The intrusives were emplaced at depths of no more than a few kilometres into Paleozoic sedimentary rocks through the underlying Grenville basement. 40Ar/39Ar analyses of amphibole yield complex age spectra in which the apparent ages decrease with increasing heating temperature. Conventional K–Ar analyses of amphibole, pyroxene, and feldspar yield anomalously old dates. These data indicate the presence of small yet significant amounts (up to 8 × 10−11 mol/g) of excess 40Ar. The quantities of excess argon are variable between rocks and minerals from the same rock. In contrast, biotites yield concordant plateaus and consistent ages ranging only from 124.1 to 124.6 Ma. These data define the age of the complex (124.4 ± 1.2 Ma) and imply a short time span of emplacement of about 0.5 Ma or less. The restricted time interval suggests the various lithologies are cogenetic. The nature of the amphibole 40Ar/39Ar spectra indicates that excess 40Ar was acquired after crystallization. The 40Ar could have been present in the initial magma either from crustal assimilation or from a mantle source. Alternatively, it could have been derived from outgassing of older crustal rocks. The occurrence of excess 40Ar in epizonal plutons may be a more widespread phenomenon than commonly recognized.