ABSTRACTThe following investigation includes studies of the two ternary systems K2O2‐Al2O3‐SiO2 and Na2O‐Al2O2‐SiO2.In each system two areas of low fusibility were found; one including members high in alkali, low in silica, and low in alumina; the other including those high in silica, low in alkali, and low in alumina. In both systems the high silica area was the one studied in detail as being of more practical ceramic value.On each triaxial three eutectics (two binary and one ternary) were established. They had the following compositions:The terms “eutectic” and “deformation eutectic” are used in this paper to designate those compositions which require the lowest temperature to complete deformation when uniformly heated with other compositions in the manner outlined in the following studies. Deformation was judged by the same method used with standard pyrometric cones.