The Methylphenanthrene Index (MPI-1) (Radke and Welte, Advances in Organic Geochemistry 1981, pp. 504–512, Wiley, Chichester, 1983) has been calibrated against vitrinite reflectance using Australian Carboniferous, Permian, Jurassic and Tertiary sediments and coals containing terrestrial organic matter. The correlation obtained was slightly different from that found previously and has resulted in an extension of the relationship to lower maturation levels (vitrinite reflectance = 0.5% R o). In this sample set the Methylphenanthrene Distribution Factor (Kvaldheim et al., Geochim. Cosmochim. Acta 51, 1883–1888, 1987) showed a poorer correlation to vitrinite reflectance than MPI-1. Hydrogen-rich organic matter from an immature marine sequence gave anomalously high Methylphenanthrene Indices and confirms a source control on the index at low maturation levels. The index has been used to confirm the low maturity of some Australian terrestrial oils and to determine the degree of maturation of organic matter and oils in Proterozoic sediments.