This paper introduces a robust 2D localization system for passive UHF RFID transponders based on phase evaluation where no calibration is required. To handle the ambiguity caused by phase measurement, several transponders are arranged to form a uniform linear array whose position is estimated simultaneously with its orientation based on phase-difference-of-arrival evaluation. 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 a commercial off-the-shelf RFID reader, conventional passive EPCglobal Class-1 Gen-2 UHF RFID tags, several transceivers, baseband hardware, and signal processing was built. Measurements were carried out in an indoor office environment, and the experimental results showed robust localization with a root-mean-square deviation of 0.10 m.