The magnitude of the gray levels in an image obtained by an acoustric microscope can be exploited to measure the acoustic impedance of a sample with the spatial resolution of the instrument. Theoretical considerations show the functional dependence of the maximal amplitude of specular reflection from material constants, which is confirmed by experiments. It turns out that for these amplitude values the opening angle of the lens plays a minor role. For a quantitative evaluation of the impedance, calibration by a known object material is necessary. ©1995 American Institute of Physics.