is a trigonal pyramidal molecule with C molecular symmetry. The length of the P−H bond is 1.42 Å, the H−P−H bond angles are 93.5°. The dipole moment is 0.58 D, which increases with substitution of methyl groups in the series: , 1.10 D; , 1.23 D; , 1.19 D. In contrast, the dipole moments of amines decrease with substitution, starting with ammonia, which has a dipole moment of 1.47 D. The low dipole moment and almost orthogonal bond angles lead to the conclusion that in  the P−H bonds are almost entirely  and phosphorus 3s orbital contributes little to the P-H bonding. For this reason, the lone pair on phosphorus is predominantly formed by the 3s orbital of phosphorus. The upfield chemical shift of it P NMR signal accords with the conclusion that the lone pair electrons occupy the 3s orbital (Fluck, 1973). This electronic structure leads to a lack of nucleophilicity in general and lack of basicity in particular (pK = –14), as well as an ability to form only weak hydrogen bonds.
The aqueous solubility of  is slight: 0.22 cm of gas dissolves in 1 cm of water. Phosphine dissolves more readily in non-polar solvents than in water because of the non-polar P−H bonds. It is technically amphoteric in water, but acid and base activity is poor. Proton exchange proceeds via a phosphonium () ion in acidic solutions and via phosphanide () at high pH, with equilibrium constants K =  and K = . Phosphine reacts with water only at high pressure and temperature, producing phosphoric acid and hydrogen:
Burning phosphine in the air produces phosphoric acid):