The invention relates to a method and arrangement for passivating the surface of semiconductor samples in which, simultaneously to passivation, the life-time of free carriers is also determined by illuminating the semiconductor sample by a light pulse of higher energy than the forbidden band of the material, and the time function of the resistance change occurring in the semiconductor as a result of illumination is measured advantageously by a microwave reflectometer, and the life-time of carriers is determined as the characteristic time constant of the process. The essence of the method lies in that, the surface part of the semiconductor sample to be measured is continuously electrically charged. The arrangement comprises a microwave reflectometer (2), a signal processing unit (3), and a laser light source (4) illuminating the semiconductor sample (1). The surface of the semiconductor sample (1) charged by ions generated by corona generators (5, 6) simultaneously with the measurement.