In this comprehensive study on unsaturated phosphorus compounds of the type R-P=CR 2 , R-P=N-R, and R-P= P-R, we investigated how the electronic, spectroscopic, and geometric parameters are influenced by different substituents by means of ab initio and DFT methods. The parent systems are studied with highly accurate theoretical methods and used to benchmark DFT methods. The substituent effects are rationalized based on orbital analysis of DFT calculations for phos- [a]