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Substitution of SiO 2 in the ternary sodium borosilicate system with Ta20~ was found to produce glasses, which after heat treatment separated into immiscible microphases, one of which was water soluble. The structure of the leached material after heat treatment was a well developed low temperature form of Ta205. After firing at temperatures between 1100 and 1 550 ~ X-ray diffraction analysis showed the presence of low and high temperature forms of Ta20~ and of orthorhombic Na2TasO21. The high solubility of up to 40 wt% TasO s in the sodium-borate matrix resulting in clear glasses is of practical interest. The specific surface areas of the leached materials ranged between 5.54 and 35.57 m s g-1 while in an additionally AI203 doped material the value of 307 m sg-1 was measured. Mean pore radii of interconnected pores were calculated to be between 18.63 to 41.1 2 nm in the TasO5-rich materials while the additional AI203 doping decreased the value to 2.71 nm. A sintering temperature between 1 500 and 1 550 ~ is estimated from void volume measurements after a series of firing steps at temperatures between 11 00 and 1 550 ~ were undertaken.