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As a part of our teams’ efforts in developing ultra-high resolution PET systems, this paper investigates the performance of 1 mm resolution Cadmium Zinc Telluride (CZT) detectors capable of positioning the 3-D coordinates of individual 511 keV photon interactions. The detectors are 40 mm × 40 mm × 5 mm monolithic CZT crystals that employ a novel cross-strip readout with interspersed steering electrodes to obtain high spatial and energy resolution. The study found the best-case single anode FWHM energy resolution of 2.7±0.2% at 511 keV, and neighborsummed FWHM energy resolution of approximately 4.64±0.35% at 511 keV. Improved resolution is expected with properly shielded front-end electronics. Measurements made using a collimated beam established the efficacy of the steering electrodes in facilitating full charge collection across anodes, as well as a spatial resolution of 1 mm in the direction perpendicular to the electrode planes. Finally, measurements based on coincidence electronic collimation demonstrated a spatial resolution of 1 mm transverse to the anodes -as expected from the 1 mm anode pitch. These findings indicate that the CZT-based detector concept has excellent performance and shows great promise for a high resolution PET system.