Phase relationships in the systems Li2O-B2O3 and Cs2O-B2O3 were studied by the oscillation method of phase analysis (OPA) using 25 and 40 compositions, respectively. The crystalline phases observed were (a) Li2O . B2O3, which melts congruently at 846 +/- 2 degrees C and Cs2O . B2O3, which melts congruently at 714 +/- 2 degrees C, (b) Li2O . 2B(2)O(3), which melts congruently at 914 +/- 2 degrees C and Cs2O . 2B(2)O(3), which melts congruently at 685 +/- 2 degrees C, (c) 2Li(2)O . 5B(2)O(3), which melts congruently at 894 +/- 2 degrees C and incongruently at 864 +/- 2 degrees C, and a new compound, 2Cs(2)O . 5B(2)O(3), which melts incongruently at 657 +/- 2 degrees C and dissociates below 607 +/- 5 degrees C, (d) Li2O . 3B(2)O(3), which melts congruently at 850 +/- 2 degrees C and incongruently at 834 +/- 2 degrees C, and Cs2O . 3B(2)O(3), which melts congruently at 826 +/- 2 degrees C, (e) Li2O . 4B(2)O(3), which melts incongruently at 630 +/-20 degrees C and Cs2O . 4B(2)O(3), which melts incongruently at 657 +/-10 degrees C, (f) Cs2O . 5B(2)O(3), which melts congruently at 663 +/-5 degrees C.