A portion of the system CaO‐SiO2has been reinvestigated by high‐temperature microscopy. Direct observations at the liquidus establish a primary‐phase region for tricalcium silicate, which is shown in a revised phase diagram to melt incongruently. The tricalcium silicate primary‐phase field extends from the eutectic with dicalcium silicate at 2050 °C., 69.5% CaO, to the incongruent boundary with lime at 2070°C., 71.5% CaO, with a possible error in composition of ± 0.25%. The estimated accuracy of these temperatures is ±3° relative to one another, and ± 10° relative to the melting points of alumina and dicalcium silicate, for which the values 2040°C. and 2130°C. taken from the literature h