The optical phase of a beam diffracted in an acousto-optic interaction may be directly controlled with the acoustic phase, but limitations occur in the case of commonly used AO devices, based on strong anisotropic AO interaction. In our study we present measurement and theoretical calculation of the phase shift of a laser beam diffracted in an AO deflector. The optical phase is a function of the acoustic and optical frequency and of the relative spatial positions of the interacting beams. We measured the phase shift with a heterodyne setup, where we measured the phase of the heterodyne signal relative to a fixed reference. We found that the phase shift introduced by the AO crystal is linear with the sound frequency, its gradient is of 10-20rad/MHz and depends mainly on the optical beam's distance from the transducer. We show that this phase shift can be derived theoretically from the momentum matching condition that must be fulfilled at each optical and acoustic frequency pair.