Thermo-optic coefficients anisotropy was characterized in monoclinic potassium (rare-earth) double tungstates KGd(WO4)2 and KY(WO4)2 in the spectral range of 0.44-0.63 μm by a modified minimum deviation method. This approach takes into account the changes in the shape and dimensions of the prismatic sample caused by the anisotropic thermal expansion effect under uniform heating together with the conventional measurements of minimum deviation angle. At room temperature at the wavelength of 633 nm, principal refractive indices for KGdW are n(p)=2.0135, n(m)=2.0458, and n(g)=2.0860, and for KYW they are n(p)=1.9979, n(m)=2.0396, and n(g)=2.0869. Optical axes position for KGdW (KYW) crystals is N(g) ± 42.60 (N(g) ± 44.1°) in the N(p)-N(g) plane. For both crystals, all the thermo-optic coefficients are negative and equal dn(p)/dT=-10.6, dn(m)/dT=-8.4, and dn(g)/dT=-15.2 for KGdW, and dn(p)/dT=-10.1, dn(m)/dT=-7.3, and dn(g)/dT=-8.4 for KYW [10(-6) K(-1)] (at 633 nm).