When studying the phenomenon of the pulling into step of a synchronous induction motor from the mathematical point of view, it is usually assumed that the synchronizing torque is a sinusoidal function of the angular distance between the stator and rotor fields. The following research points out that this is erroneous for several reasons, the most important of which is the effect of armature reaction. This sets up a double-frequency component in the synchronizing torque which very considerably modifies the conditions during synchronizing. The synchronizing torque of a salient-pole motor also possesses a double-frequency component but it is produced differently from that of the non-salient-pole motor. It is shown that the operation of the motor when this component is taken into account is amenable to mathematical solution. An experimental investigation carried out on a small induction motor run as a synchronous motor is also described.