A comparison of published metabasite amphibole analyses from medium and low-pressure metamorphic terrains reveals that there is no systematic variation in Na, NaM4, Al or AlVI as a function of pressure. This may be due to blurring of the differences by variation in oxidation state, or by analytical differences between laboratories. It is not due to variable Mg/Fe in whole rocks. Differences that can be recognised are generally higher Ti/Al ratios in the low-pressure amphiboles, and a very poorly developed compositional gap between actinolite and hornblende compared with a well-developed gap at medium pressures. These features, together with the relatively low-grade appearance of calcic plagioclase at low pressures, provide the best means of distinguishing metabasites from the two facies series. All three features can be explained by the configuration of cation-exchange equilibria at the greenschist/amphibolite facies boundary. Enrichment in Ti at low-pressures is due to the positive slope of reactions partitioning Ti into the amphibole. The composition gap in amphiboles at medium-pressure is due to overstepping of the tschermakite-enriching equilibrium. At low pressures this overstepping still occurs, but the equilibrium tschermakite-content in the amphibole is much lower for a given amount of overstepping. The relatively low-grade appearance of oligoclase at low pressures is due to convergence of the tschermakite and anorthite-enriching equilibria with decreasing pressure.