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The paper investigates the role of primary magmatic phases in the fractionation and concentration of PGE in Archaean mafic and ultramafic systems. The composition of chromites and olivines in sulphur-poor (S<0.6?wt%) komatiites from the Agnew-Wiluna Belt (Western Australia), and of chromite concentrated from komatiitic basalt, ferropicritic basalt and tholeiitic basalt from the Abitibi Belt (Canada) were analysed. The results of laser ablation ICP-MS analyses show that PGE-bearing alloys are not stable in crystallising komatiite and that ruthenium is soluble in chromite during crystallisation. Conversely, analyses of chromites separated from Theo’s Flow tholeiitic basalt indicate that Ir-Os-(±Pt) enrichments (>200?ppb) reflect the presence of PGM. Chromites from Fred’s Flow komatiitic basalt contain Ir-rich clusters, whereas Pt enrichments (>370?ppb) in Boston Creek ferropicritic basalt reflect the presence of Pt-rich compounds. The presence of PGE-bearing alloys in Theo’s Flow and Fred’s Flow is due to late S-supersaturation, whereas the presence of Pt-rich compounds in Boston Creek Flow reflects high state of melt oxidisation. The lack of PGE-bearing alloys in the olivines and chromites of komatiites can be explained by thermal instability of PGM, depletion in PGE at the mantle source, early S-supersaturation, the oxidisation conditions of the melt, or a combination of these factors. [ABSTRACT FROM AUTHOR]