Eu-151-Mossbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the EuyM1-yO2-x (M = Th and U) systems over the entire composition range of 0 <= y <= 1.0. The XRD results of the Eu-Th system showed that a very wide defect-fluorite (DF) type phase in which oxygen vacancies (V-O) are disordered (x = y/2) is formed for 0 <= y < 0.5 and that two-phase regions sandwitching a narrow C-type (C) single phase around y approximate to 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y < 1.0 (C + B-type (monoclinic) Eu2O3). The Mossbauer results show that the isomer shifts (ISs) of Eu3+ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O2- around Eu3+ with increasing y (CN = 8 - 2y) (x = y/2) results in the decrease of the average Eu-O bond length, which is due to the decrease of repulsion force between O2- anions. This result confirms that the IS of Eu3+ correlates well with the average Eu-O bond length in oxide systems. For the Eu-U system, the lattice parameter, a(0), of the system decreases almost linearly with y, in accordance with the calculated a(0) versus y curve for the oxygen-stoichiometric (i.e. x = 0) fluorite-type dioxide (CN = 8). The ISs of Eu3+ in this composition range remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu2Zr2O7 and Eu2Hf2O7 (y = 0.5)) with O2--eight-fold coordinated Eu3+(CN = 8). (c) 2005 Elsevier Ltd. All rights reserved.