Phosphaalkenes (MesP=CRR': R = R' = Ph ( 1a); R = R' = 4-FC6H4 (1b); R) Ph, R' = 4-FC6H4 (1c); R = R' = 4-OMeC6H4 (1d); R) Ph, R' = 4-OMeC6H4 (1e); R) Ph, R' = 2-pyridyl ( 1f)) are prepared from the reaction of MesP(SiMe3)(2) and O=CRR' in the presence of a trace of KOH or NaOH. The base-catalyzed phospha-Peterson reaction is quantitated by NMR spectroscopy, and isolated yields of phosphaalkene between 40 and 70% are obtained after vacuum distillation and/or recrystallization. The asymmetrically substituted phosphaalkenes ( 1c, 1e, 1f) form as 1: 1 mixtures of E and Z isomers; however, X-ray crystallography reveals that the E isomers crystallize preferentially. Interestingly, E-1e and E-1f readily isomerize in solution in the dark, although the rate of isomerization is much faster when samples are exposed to light. X-ray crystal structures of 1b, E- 1e, and E- 1f reveal that the P=C bond lengths ( average of 1.70 angstrom) are in the long end of the range typically found in phosphaalkenes (1.61 - 1.71 angstrom). Attempts to prepare isolable P-adamantyl phosphaalkenes following this route were unsuccessful. Although AdP=CPh2 (2a) is detected by P-31 NMR spectroscopy, attempts to isolate this species afforded the 1,2-diphosphetane (AdPCPh(2))(2) ( 3a), which was characterized by X-ray crystallography.