Ab initio theoretical calculations have been used to study the influence of phosphorus substituents, Y, on the tautomerism between the vinylphosphine XP(H)C(CH3)=CH2 and the phosphaalkene XP=C(CH3)(2) (X = H, F, Cl, Br, OH, and Ar-F; Ar-F = 2,6-(CF3)(2)C6H3) and on the acidity of the aforementioned vinylphosphine. The stabilization of the phosphaalkene and the increased acidity of the vinylphosphine by Ar-F are possible factors in the successful synthesis of certain isolable phosphaalkenes. In this work, the properties of Ar-F are assessed theoretically. Density functional theory using the B3LYP functional has been used for all substituents. In addition, coupled-cluster singles and doubles with noniterative triples (CCSD(T)) has been used for X = H, F, Cl, Br, and OH. The phosphaalkene is favored over the vinylphosphine for all substituents, with F having the strongest stabilizing effect. Cl, Br, and OH have stronger stabilizing effects than Ar-F. In contrast, the most acidic vinyl phosphine is that with Ar-F. To aid in the interpretation and analysis of future experimental work, CCSD(T) calculations were used to provide structures and vibrational frequencies for the series XP=C(CH3)(2) (X = H, F, Cl, Br). The influence of the substituent on geometries and C=P and X-P stretching frequencies was examined, and comparisons were made with the CH2=PX series.