A biocidal composition comprising 2,2-dibromomalonamide and an oxidizing biocide, and its use for the control of microorganisms in aqueous and water-containing systems. The coil is connected through a switch to an operating voltage potential or to a reference potential. Therefore, the just-described reflected portion of the probe beam is interfered with a reference beam formed by a portion of probe beam reflected by the front surface of the semiconductor material, and phase and amplitude of the interference signal resulting therefrom are both measured.