Chalcogen microclusters have been produced in free space by means of the supersonic jet expansion method. The clusters were ionized by electron impact and analyzed with a Wiley-Mclaren type time-of-flight mass spectrometer. Se n + clusters up to n = 8 were detected, when selenium in the cluster source was heated up to 400 °C. The mass spectrum near the appearance potential indicates that Se 2 , Se 5 and Se 6 are dominant species in the cluster beam. When the cluster beam was irradiated by 700 mW laser light with wavelength of 488 nm, keeping the temperature of cluster source constant, the intensity of Se 5 + decreased.