Results on investigation of thermal effects which appear during operation of an acousto-optic deflector are presented in the paper. Influence of the effects on characteristics of the scanning device is evaluated. For a case of the tellurium dioxide deflector, it is proved that the most principal thermal effect consists in an undesirable spatial shift of a scanning optical beam. The shift leads to the optical beam positioning error which in a typical deflector may be of dozens of resolvable spots. A method of the thermal shifts compensation based on acoustic frequency tuning is proposed. For various regimes of the deflector operation, laws of the frequency tuning are found thus providing the compensation of the shifts with the required precision.