The Larvik plutonic complex is an intrusive suite within the Palaeozoic Oslo Rift, Norway, consisting of ten individual ring sections of monzonite (larvikite) and lardalite. The Bjønnes nepheline syenite is a relatively small intrusive body within nepheline-normative larvikite and prior to this research, very little was known about the intrusion. The Bjønnes syenite is texturally and chemically heterogeneous, locally becoming mildly peralkaline ((Na+K)/Al = 0.91 – 1.06). These peralkaline nepheline syenites display agpaitic mineralogy, including minerals such as eudialyte and aenigmatite. The miaskitic-agpaitic transition is variable, most likely influenced by other factors beyond alkalinity. Agpaitic minerals were previously thought to be confined to nepheline syenite pegmatites in the Larvik plutonic complex. Electron microprobe analysis of mafic minerals showed a compositional range from diopsidic to almost pure aegirine (Ae9Di59Hd32 – Ae99Di1Hd0). Clinopyroxenes produce a single evolutionary trend away from a primitive composition through zonation and increased alkalinity. Secondary clinopyroxene that replaced amphibole as a post-magmatic hydrothermal phase has the most evolved composition. Amphibole compositions show a similar increase in Na and Fe components as alkalinity increases. Geochemical whole rock data shows that the Bjønnes syenite has been depleted in elements such as P, Sr and Eu, removed during extensive fractionation of plagioclase and apatite from a source melt. Incompatible trace element ratios suggest that the Bjønnes syenite originated from a homogeneous mantle source compatible with the source of the larvikites. Complex field relationships and major element comparisons suggest that the Bjønnes syenite intruded hot larvikite of ring section VI and most likely originated from the same, fractionated melt source. Widespread gneissic texture relates to syn-magmatic deformation, possibly indicating emplacement via faulting. Nepheline syenite dykes in the surrounding areas are likely related to the Bjønnes syenite based on similarities in texture, filed relationships and geochemistry.