This paper presents a 2-D localization system for passive UHF RFID tags on the basis of analysis of backscattered transponder signals. The maximum likelihood position estimator we introduce combines inherently phase and amplitude evaluation, thus enabling accurate and robust localization. A MIMO system in which, sequentially, each frontend is configured to work as a transmitter while the remaining frontends serving as receivers is used for position estimation. For proof of concept, we built 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 transceiver frontends, baseband hardware, and signal processing. Measurements were carried out in an indoor office environment, and the experimental results showed accurate localization with a root-mean-square error of 0.054m and a median error of 0.011 m.