Instantaneous temperature measurements at the interface between a solid wall and a thin, unsteady liquid film are performed using thermoreflectance, a non-intrusive optical technique with high temporal resolution. A laser beam is directed at a wall-liquid interface and the intensity of the light reflected at that interface is measured by a photodiode. The intensity of the reflected light varies with the index of refraction of the liquid at the wall. The index of refraction is a function of temperature, which enables the instantaneous measurement of the wall temperature. In the presence of thin liquid films, reflections from the liquid-vapor interface at the free surface of the film generate noise in the measurements. We demonstrate that orienting the laser beam at a large incident angle, close to total internal reflection, minimizes noise from the liquid-vapor interface while increasing the sensitivity of the measurement. The thermoreflectance technique is validated in an unsteady two-phase annular flow. Measurements of temperature fluctuations less than 1 K in amplitude are achieved, with an uncertainty of 0.1 K.

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