meso- and racemic-1,2-diethyl-1,2-dimethyldiphenyldisilane (2a) and (2b) were synthesized respectively by hydrogenation of meso- and racemic-1,2-diethynyl-1,2-dimethyldiphenyldisilane in the presence of a IrCl(CO)(PPh(3))(2) catalyst. Irradiation of 2a with a low-pressure mercury lamp in the presence of isobutene in hexane proceeded with high diastereospecificity to give (R,S)- and (S,R)-2-(isobutylethylmethylsilyl)-1-(ethylmethylphenylsilyl)benzene in 77% yield. Similar irradiation of 2b with isobutene produced (R,R)- and (S,S)-isomer. The photolysis of 2a and 2b in the presence of 1,1-diphenylethylene also proceeded diastereospecifically to give the respective adducts. With 2,3-dimethylbutadiene, 2a and 2b produced the corresponding adducts, whose spectrometric analysis showed the presence of two diastereomers. Results on theoretical studies which were carried out using PhSiH(2)SiH(3) as a model have also been reported.