A method of controlling transmittance of an electrochromic device is presented. The electrochromic device to be controlled has a first and a second electron conducting layer, a first electrochromic layer covering the first electron conducting layer, a counter electrode layer covering the second electron conducting layer and an electrolyte layer laminated between the first electrochromic layer and the counter electrode layer. The method includes applying (212) of a sequence of voltage pulses between the first and second electron conducting layers and providing of an open circuit between the first and second electron conducting layers between the applied voltage pulses. The method is characterised by measuring (214) a voltage between the first and second electron conducting layers during a period of the open circuit and controlling (220) a pulse parameter of the voltage pulses dependent on the measured voltage, where the pulse parameter is one of pulse duration and pulse voltage.