Zirconium diboride (ZrB2) was densified (> 98% relative density) at temperatures as low as 1850 degrees C by pressureless sintering. Sintering was activated by removing oxide impurities (B2O3 and ZrO2) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (similar to 150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B4C were evaluated as additives to facilitate the removal of ZrO2. Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB2 (surface area similar to 1 m(2)/g) or attrition-milled ZrB2 (surface area similar to 7.5 m(2)/g) with WC and/or B4C as a sintering aid. ZrB2 containing only WC could be sintered to similar to 95% relative density in 4 h at 2050 degrees C under vacuum. In contrast, the addition of B4C allowed for sintering to > 98% relative density in 1 h at 1850 degrees C under vacuum.