The monomer and excimer fluorescence quantum yields of well-defined poly(dimethylsiloxane-co-diphenylsiloxane)s with different diphenylsiloxane (Ph2SiO) contents have been determined, along with those of 1,1,3,3-tetraphenyl-1,3-diniethyldisiloxane and 1,1,3,3,5,5-hexaphenyltrisiloxane-1,5-diol used as model compounds, in a dilute organic solvent at different temperatures. The measured fluorescence quantum yields of the copolymers are correlated with the fraction of the -(CH3)(2)SiO-(Ph2SiO)(n)-(CH3)(2)SiO- structures. The monomer fluorescence yield for copolymers with low Ph2SiO contents is dominated mainly by the isolated -(CH3)(2)SiO-(Ph2SiO)-(CH3)(2)SiO- unit, and the apparent mean binding energy of the excimer does not increase significantly with increasing Ph2SiO content.