Abstract: Relict clinopyroxenes from metadolerites of the Early Permian Petroi Metabasalt formation, studied by electron microprobe, show a limited compositional range near the diopside-augite boundary in the pyroxene quadrilateral. Clinopyroxene analyses from three metadolerites, grouped in approximately equal Fs contents, define an overall smooth trend between Fs₁₀ and Fs₁₆. This is typical of clinopyroxenes from mildly alkaline basic magmas. Pyroxene stoichiometry suggests high Fe³⁺ contents (0.04 to 0.20 a.f.u.), which with high Al (0.10 to 0.31 a.f.u.) and Ti (0.03 to 0.08 a.f.u.) implies that CaTiAl₂O₆ and CaFe³⁺AlSiO₆ are important ‘other components’. Relative Al_z in CaFe³⁺AlSiO₆ decreases and consequently CaTiAl₂O₆ increases with progressive fractionation. This, with the Fe²⁺:Fe³⁺ ratios in the Petroi clinopyroxenes, suggests falling ƒ_O2 in the magma with fractionation. The ƒ_O2 controlled entry of Al_z, Ti_y and Na_x into the clinopyroxenes and hence the Petroi clinopyroxene trend.