Abstract An analysis of the existence of athermal properties for optically biaxial crystals KGd(WO 4 ) 2 and KLu(WO 4 ) 2 is performed and compared with that of KY(WO 4 ) 2 for monolithic and laser cavity configurations. Short-wavelength limits for athermal behavior in the principal planes of optical indicatrix are found. In KGd(WO 4 ) 2 they lie near or in the absorption band. In KLu(WO 4 ) 2 , one of the two isonomal waves and both isonomal waves in the N p –N m principal plane of KGd(WO 4 ) 2 for the laser cavity configuration have also long-wavelength limits. It is shown that for some wavelengths (one or two in the visible spectrum) and light wave polarization, the optical indicatrix axes can be athermal. In KLuW, for the monolithic configuration, the N p axis is athermal at three wavelengths, two of which lie in the IR region (0.843 and 1.975 μm). All-space athermal directions are determined for KLu(WO 4 ) 2 at 2 μm. For the monolithic configuration, they form a cone with its axis parallel to the N p axis and oval cross-section.