A range of lambda(3)-phosphaalkynes P=C-R (R = F, Cl, O-, OLi, OH, OSiMe3, S-, SLi, SH, Me, SiMe3, H)(1) has been geometry-optimized at the HF, MP2, and CCSD(T) level of theory. Vibrational frequencies as well as P-31 and C-13 NMR chemical shifts were calculated and compared to experimental values of the large variety of lambda(3)- phosphaalkynes prepared over several years via differing synthetic routes. With a few obvious exceptions, a good agreement of calculated and experimental parameters was found. This also opens up the possibility to make predictions on spectroscopic data of compounds not yet synthesized or fully characterized, e.g., PCOSiMe3 and PCSH. The bonding situation especially of the negatively charged species P=C-O- and P=C-S- was studied via NBO analyses to determine the degree of phosphaalkyne and heterocumulene contributions to the mesomeric anions.