This paper introduces a 2D localization system for passive UHF RFID tags based on phase evaluation of the backscattered transponder signals. To increase the accuracy and robustness of the position estimates an inverse synthetic aperture radar approach is performed. A MIMO system where paths take turns to act as transmitter with the remaining paths serving as receivers, is used to enable the position estimation. For proof of concept, a local position measurement system demonstrator comprising an RFID reader, passive EPC Gen 2 RFID tags, several transceivers, baseband hardware, and signal processing was built. Measurements were carried out in an indoor office environment, which was surrounded by drywalls and concrete floor and ceiling, and the experimental results show a robust and accurate localization with a root-mean-square deviation below 0.1 and a median error of 0.01m.