A series of phosphaalkenes, E- ArPdC(H)Ar' (Ar) 2,6-Mes(2)C(6)H(3), Ar' = Ph (1a); Ar = 2,6-Mes(2)C(6)H(3), Ar' = p-C6H4Br (2a); Ar = 4-Br-2,6-Mes(2)C(6)H(2), Ar' = Ph (3a); Ar = 4-Br-2,6-Mes(2)C(6)H(2), Ar' = p-C6H4Br (4a)) have been prepared by phospha-Wittig reactions and characterized. Exposure of these materials either to room light over an extended period of time (days) or to UV light (hours) produced equilibrium mixtures of the E and Z isomers (1b-4b) as indicated by H-1 and P-31 NMR spectroscopy. The structures of compounds 4a and 4b were determined by single-crystal X-ray diffraction methods. Variable-temperature 1H NMR studies of 4b indicate hindered rotation about the P-C-Ar bond, with Delta H-double dagger = 13.8 kcal/mol and Delta S-double dagger = 1.3 eu. The electronic structures of E- and Z-PhP=C(H)Ph have been examined using density functional theory.