Ab initio molecular orbital calculations at the MP2/6-31G* level of the structure, vibrational spectra, and themochemical properties of triphospha[1.1.1]propellane (4), diphosphabicyclo[1.1.0]butane (5), and phosphirane (6) have been carried out and compared with those of their all-carbon analogues. 4 possesses a ring strain even smaller than that of [1.1.1]propellane and is characterized by a bridgehead C-C distance of 1.63 angstrom and a heat of formation (DELTAH-degrees(f,0)) of 560 +/- 20 kJ/mol. The observable IR bands of 4 are centered in the regions of 2291, 947, 755, and 459 cm-1, while the first vertical UV absorption of 4 is expected at 5.3 +/- 0.5 eV. Although the C-C bridgehead distance in triphospha[1.1.1]propellane (4) is rather long, it has been shown by atoms-in-molecule calculations that a bridgehead bond exits and has a pronounced covalent character.