The photoirradiation of trans- and cis-poly(dimethylsilylenephenylenevinylene)s gave cis-rich mixtures at equilibrium states. The degree of the photoisomerization could be exactly evaluated by comparing the UV spectra of the photoirradiated solutions with those of the trans and cis polymers. The geometric configuration of the trans and cis polymers was thermally stable and hardly changed even though they were heated. The trans and cis polymers exhibited different emission properties; e.g., trans polymer: lambda (max) = 400 nm, quantum yield = 3.4 x 10(-3); cis polymer: lambda (max) = 380 nm, quantum yield = 1.5 x 10(-3).