Synthesis and thermal behavior investigation of Bi1-xYbxO1.5 materials (0.166 less than or equal to x less than or equal to 0.666) have been realized, in order to obtain the low temperature equilibrium phases. A plot of the Bi2O3-Yb2O3 equilibrium phase diagram is proposed. Two new compounds, structurally related to the delta-Bi2O3 variety, are evidenced. Bi17Yb7O36 (x = 0.292) crystallizes in an orthorhombic cell with a = 16.259(2), b = 10.705(1), c = 5.487(1) Angstrom. It transforms into the cubic delta-Bi2O3 type variety over 855 degreesC. Only this cubic form, easily quenchable in a metastable form, was reported up to now as a composition of a 6 solid solution domain (6). A narrow domain of orthorhombic solid solution (epsilon) could exist (0.275 less than or equal to x less than or equal to 0.305). A second compound BiYbO3 crystallizes in a triclinic cell with a = 8.487(1) b = 8.419(1), c = 9.928(1) Angstrom, alpha = 86.03(1), beta = 111.29(1), gamma = 74.25(1)degrees. At 970 degreesC, a peritectoid transformation into a mixture of delta and Yb2O3-based solid solution (Yb2O3-s.s.) occurs; this latter disappears at about 1200 degreesC. This last event is easily reversible on cooling, whereas the first one is very sluggish. Consequently, for the composition domain x > 0.35, the samples cooled from the high temperature are generally constituted of a mixture of delta-type phase and Yb2O3-s.s. (C) 2004 Elsevier Ltd. All rights reserved.