Using product analysis and time-resolved laser flash photolysis techniques, the photochemistry at lambda(exc) approximate to 250 and 310 nm (exe = excitation) of 1-(p-R-phenyl)-2-(2,2’-biphenyldiyl)vinyl halides (R = H, Me, MeO) in methanol and acetonitrile at room temperature was studied. The title compounds undergo photoheterolysis and photohomolysis to give vinyl cations =C+- (carbenium ions) and vinyl radicals =C-.-. The cation : radical ratio increases with electron-donating strength of the substituent R, H < Me < MeO, and with anionic leaving group power of the halide, F- < Cl- < Br- < I-, indicating that the cleavage of the C-X bond to yield cation and halide anion proceeds, in the rate-determining step, by heterolysis and not by homolysis followed by electron transfer in the radical pair. The cation:radical ratio is solvent dependent : e.g., for the vinyl bromide with R = MeO and with lambda(exc) = 308 nm, in CH2Cl2 as solvent, only radical is observed, in comparison with only cation in the much more polar solvent acetonitrile. In acetonitrile-methanol mixtures, the absolute yields of both cation and radical go through a maximum as the methanol content is increased, however, the cation:radical ratio decreases continuously with increasing [MeOH]. For a particular R, the quantum yield for C-X bond cleavage is higher at approximate to 250 than at approximate to 310 nm. Also, the cation:radical ratio is wavelength-dependent : at lambda(exc) approximate to 310 nm there is relatively more heterolysis than at lambda(exc),,, x 250 nm. Rate constants for reaction of the cations with nucleophiles were determined in acetonitrile. Those for reaction of the cation with R = MeO (lifetime in acetonitrile 7 ys) with anionic nucleophiles such as the halides are at the diffusion limit in this solvent, approximate to 2 x 10(10) M(-1) s(-1), whereas those for reaction with water, alcohols, and cyclic ethers are of the order 10(5)-10(6) M(-1) s(-1). The less stabilized cation (R = Me) reacts with alcohols faster by the factor approximate to 100, and it decays in acetonitrile approximate to 100 times more rapidly. Also, 1-(p-a-phenyl)-2-dimethylvinyl bromides (R = H, Me, MeO) were photolyzed in acetonitrile. In the case of R = MeO, the cation p-CH3O-C6H4-C+=CMe(2) was seen (lifetime 770 ns), and its reactivity with alcohols (k values of 10(6)-10(7) M(-1) s(-1)) and halides (k values of 10(10) M(-1) s(-1)) was determined. The vinyl radicals =C-.- react with O-2 to yield vinylperoxyl radicals =C-O-2(.) which have absorption maxima at approximate to 390 nm.