Absorption, ESR and photoluminescence of hexakis(pentamethyldisilyl)disilane [(Me3SiMe2Si)(3)Si](2) 1 were studied. Two branched polysilyl radicals were observed by ESR spectroscopy under UV irradiation of 1. The smaller 'star' radical (Me3SiMe2Si)(3)Si 2 has half-lifetime 1 s whereas the bigger one (Me3SiMe2Si)(3)Si(Me3SiMe2Si)(2)Si' 3 has a half-lifetime about 50 min. During irradiation the reaction 1 --> 2 + 3 followed by 2 + 3 --> products (P) occur. The behavior of the 300 nm absorption of radical 3 during the irradiation was found to be similar to the radical 3 appearance measured by ESR. The silane (Me3SiMe2Si)(3)SiH was found as main product (similar to50% yield) of the photo-cleavage of the disilane 1. (C) 2003 Elsevier Science B.V. All rights reserved.