A simple dynamic technique for the determination of the absolute signs of photoelastic coefficients is described. The method consists of comparing the known phase of a standing acoustic wave with the phase of the deflection of a photoelastically refracted laser beam, the waist of which is small compared with an acoustic wavelength. Experimental verification of the technique is presented along with specific limitations. The method forms a convenient complement to the Bragg diffraction measurement of the magnitudes of photoelastic coefficients.