The synthesis and crystal structure of potassium tantalate (KTa2(O,OH)(6). 1.8H(2)O) with a defect pyrochlore structure are reported. The compound was crystallized directly from tantalum pentoxide in basic solution under hydrothermal conditions at temperatures as low as 200 degrees C. Ion exchange of this substance in acid yielded an H-type defect pyrochlore. Both highly crystalline K-type and II-type materials absorb argon and have BET surface areas of about 14 m(2/)g. The crystal structures were refined by the Rietveld least-squares method from powder X-ray diffraction data. The final R-wp and chi(2) were 7.70% and 2.79, respectively. The high thermal stability and control of acidity of these materials, along with the simple and cheap synthesis, may find many applications in sorption, catalysis, and microelectronics.