We discuss implementation and testing of phase referencing at the Palomar Testbed Interferometer. A new instrument configuration provides a coherent integration of 10 or 20 ms on a bright star while stabilizing the fringe phase of a nearby (20 00 ) and faint visual companion, allowing coherent integration times of at least 250 ms. Observations have been made of several visual binaries, including 16 Cyg AB (m K = 4.5 and m K = 4.6) and HD 173648/49 (m K = 4.3 and m K $ 5) to test the performance

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... of the technique. These measurements also demonstrate that phase-referenced visibility measurements can be calibrated at the level of 3%-7%.