The binary system Nb205--S102 has been shown to include an extensive two-liquid region over the range 5 to 80% Nb205. The minimum temperature of the two-liquid area is 1695°C. A eutectic composition occurs at 95% Nb205 and 1448°C. and another at approximately 5% Nb2O5 and 1695°C. The experimental results were obtained by the cone-fusion method. : The binary system CaO-Nb205 has been shown to include three compounds: CaO -Nb2Or which melts congruently at 1560°C; 2Ca0 -Nb20s, which melts congruently at 1576°C; and 3Ca0- Nb20s, which melts incongruently at 1560°C. Three eutectic compositions occur at 6% CaO (1362°C), 23 ( ; CaO (1492°C), and 34% CaO (1535°C). These results were obtained by the cone-fusion technique. The compound 3CaONb205 has been shown to exist in two forms : type I, face-centered cubic with a = 7.978 A, having a superlattice with a = 23.934 A, and type II, orthorhombic (pseudotetragonal, distorted cubic) with a = 11.51 A, b = 11.10 A, r = 15.98 A, having a pseudocell with a = 5.754 A, = 5.551 A, and c = 7.990 A. The conditions controlling the formation of these two forms were not determinable from the experiments conducted.