Deviations of an x-ray mirror from its ideal shape, with errors as small as one arcsec, can severely degrade its overall performance. Because the mirrors in third-generation synchrotron sources are typically exposed to extremely high power and power density, their surfaces are very susceptible to heat-induced distortions, imposing demanding constraints on both the mirror design and construction. These x-ray mirrors are routinely measured off-line for their surface roughness and figures; however, in-situ diagnostics are much more rare. Knowledge about the deformation of these mirrors under high heat load enables one to check the mirror performance, to improve upon the mirror design and construction, and to verify the thermal deformation calculations as predicted by theoretical and finite-element analysis. In response to this crucial need, we have developed at the Advanced Photon Source a portable electro- optical system for real-time, in-situ monitoring of thermally induced surface distortions of synchrotron mirrors under high heat load. We employ multipass technique to magnify (mu) rad angular (or slope) deviations to within the resolution of a commercially available CCD camera (with 9- micrometer square pixels). This system has been demonstrated to have angular resolution of less than 1 (mu) rad (or 0.2 arcsec) and long-term stability of less than 2-(mu) rad) fluctuation over a period of 12 hrs., after digital signal processing. With the present camera, the system is capable of sampling one surface profile (at 8 discrete points) about every 2 minutes and can readily be modified into an adaptive mirror system with the necessary actuator hardware. The prototype system has been installed and tested, and those results are presented.