Model 1,4-bis(styryl)benzene derivatives la,b related to poly(2-bromo-5-methoxy-1,4-phenylenevinylene) were synthesized to probe the effect of bromination on excited-state behavior of conjugated phenylenevinylene analogues. They showed solution absorption maxima at about 385 nm, without evidence of excimer formation up to 100 muM. Their fluorescence spectra showed overlapping bands at 435 and 460 run, with quantum yields in tetrahydrofuran of 0.16 and 0.21 for la and 1b, respectively. Excited-state transients were monitored by time-resolved laser flash spectroscopy at room temperature. The triplet states were characterized by absorption maxima at about 520-530 nm with lifetimes of about 0.6 mus, much longer than would be observed for prompt fluorescence states. The quantum yields of singlet to triplet-state intersystem crossing were determined to be 0.45 and 0.28 for la and 1b, respectively. The acceleration of triplet transient decay rates under increased oxygen pressure according to the Stern-Volmer law further supports the triplet-state assignments. The relatively high yields of intersystem crossing and low yields of fluorescence are attributed to a bromine-modulated heavy atom effect that enhances intersystem crossing between excited singlet and triplet states.