Five metamorphic isograds have been mapped in the calc-schists of the Riondel area, British Columbia. These isograds are based on the following model reactions:The first four of these isograds represent discontinuities in the observed mineral assemblages and are located by plotting all occurrences of the complete reactant and product assemblages. The fifth isograd occurs over a wide zone in which the anorthite component of the plagioclase in the assemblage Ep–Ac–Q–Di–Pl increases with increasing metamorphic grade, from approximately An30 in the lower grade eastern part of the area to approximately An60 in the western part.The distribution of isograds and the systematic variation in plagioclase compositions in the Ep–Ac–Q–Di–Pl assemblage is modelled using diagrams that have been calculated at 5000 bars pressure under the simplifying constraint that. These diagrams provide a basis for estimating the physical conditions of regional metamorphism in the Riondel area. Maximum temperatures exceeded 600 °C in the highest grade part of the area and the composition of the hypothetical binary fluid phase varied from approximately in the lower grade to 0.5 or 0.6 in the higher grade areas.