The dipole moments of mono-n-propylphosphine and mono-i-propylphosphine, as measured in the gas phase with a heterodyne beat apparatus, are 1.09±0.06 and 1.22±0.03 D, respectively. The moments of mono-n-propylphosphine, di-n-propylphosphine, and tri-n-propylphosphine as measured in benzene solution and corrected for solvent effect are 1.17±0.05, 1.22±0.05, and 1.16±0.08 D, respectively. The moments for mono-i-propylphosphine, di-i-propylphosphine, and tri-i-propylphosphine as measured in benzene solution and corrected for solvent effects are 1.13±0.03, 1.32±0.12, and 1.32±0.04 D, respectively. Discrepancies between gas phase and solution values are considered, and values for each series are discussed in terms of a bond polarization model.