The paper examines the kinetics of the electric field induced, first-order transition from the commensurate to the ferro-electric phase in thiourea (SC(NH2)2) at 170 K. The satellite reflection from the modulated phase, locked in with a wave vector q = 1/9b* disappears when the phase transition is induced by a static electric field Ec along the a-axis. In order to study the kinetics of the phase transition, an oscillating electric voltage of rectangular shape was applied to the sample and photon events were acquired separately for ON- and OFF-field half-periods. Results received in a series of measurements at different amplitudes between 0 and 500 V and frequencies between 0.125 and 512 Hz lead to the conclusion that a slow and a fast relaxation time play a role for the OFF → ON and the ON → OFF transitions, respectively. Further, the highest frequency at which the phase transformation still follows the switching of the electric field depends linearly on the applied field amplitude. A qualitative explanation of the experimental results is given in terms of nucleation and growth of specific ferro-electric domains.