Microwave dielectric properties of single-crystal incipient quantum ferroelectrics, KTaO3 and SrTiO3, have been measured at cryogenic temperatures. Cylindrical specimens were used as TE0n1-mode and quasi-TE011-mode dielectric resonators at temperatures ranging from 4to300K. Conductive losses of the measurement resonant structures were taken into account, both as a function of frequency and temperature, so that uncertainties in the evaluated dielectric losses were ±5%. The real permittivity was measured with an accuracy of ±0.5%. The evaluated real permittivities of KTaO3 and SrTiO3 exhibit no ferroelectric transition, and remain paraelectric down to 5K, consistent with soft-mode stabilization. Dielectric loss tangent values of KTaO3 at 3GHz were 4.2×10-5 at 5.4K, 8.9×10-5 at 77K, and 1.4×10-4 at 300K, while those of SrTiO3 were 3.4×10-3 at 5.4K, 2.4×10-4 at 77K, and 3.8×10-4 at 300K. Results of the complex permittivity measurements are compared with theoretical predictions from a modified Devonshire phenomenological model.