Irradiation of an acetonitrile solution containing dimethylbis(4-vinylphenylmethyl)silane 1a in the presence of 9,10-dicyanoanthracene leads to formation of the intramolecular photocycloadduct, 2-sila-cyclobuta[2.3]cyclophane (2a). In contrast, prolonged irradiation gave insoluble polymeric material. The photocycloreversion of 2a occurs efficiently (quantum yields exceeds unity) by use of redox-type photosensitization in the presence of magnesium perchlorate. The transient absorption spectra generated by pulse radiolysis and gamma-radiolysis show that the radical cation species generated from 1a is different from that arising from 2a.