1,2-Propadienylphosphine (allenylphosphine), H2C=C=CHPH2, has been investigated by Stark and Fourier transform microwave spectroscopy. Two rotameric forms denoted syn and gauche have been assigned. The syn form has a symmetry plane (C-s symmetry) where the lone electron pair of phosphorus points toward the double bonds. The phosphino group is rotated roughly 120degrees from this position in the gauche rotamer. The dipole moment of syn was determined to be mu(a) = 1.613(23), mu(b) = 2.347(24) mu(c) = 0 (for symmetry reasons), and mu(tot) = 2.848(28) x 10(-30) C m [0.854(8) D]. The energy difference between the two forms was found to be 2.1(4) kj/mol from relative intensity measurements with syn as the more stable conformer. Extensive quantum chemical calculations have been carried out and accurate equilibrium structures have been determined for these two rotamers, as well as for the corresponding two conformers of vinylphosphine (H2C=CHPH2).