A modified Michelson-Morley laser interferometer for measuring very small (10⁻² Å) electrostrictive displacements in low-K dielectrics is described. Problems specific to the evaluation of electrostrictive Q coefficients in low-K dielectrics, such as sample geometry, surface effects, sample warping, and Maxwell stresses are discussed. This is a subresonant method, so the frequency limitations (∼ 160 kHz, upper limit) are established for typical ceramic samples. Measurements of M_ij on fluorites (CaF₂, BaF₂,SrF₂), a fluoride perovskite (KMnF₃), Al₂O₃, BeO, LiF, spodumene glass (Li₂O · Al₂O₃ · 4SiO₂), calcite (CaCO₃) and MgO along with a glass ceramic sample (Corning Glass 888VE) and a relaxor ferroelectric (3%Ca/PMN) are reported. Results from this method are compared to a set of converse method measurements on these materials presented earlier. Excellent agreement between the two methods is obtained for all samples except the alkali halides. This establishes reliable and accurate values for the Q coefficients of these samples.