a colourless, flammable, extremely toxic gas, with a disagreeable, garlic-like odour. Phosphine is formed by the action of a strong base or hot water on white phosphorus or by the reaction of water with calcium phosphide (Ca₃P₂). Phosphine is structurally similar to ammonia (NH₃), but phosphine is a much poorer solvent than ammonia and is much less soluble in water.

Organic compounds with bonds between phosphorus and carbon or hydrogen are named as derivatives of phosphine; in primary, secondary, and tertiary phosphines, one, two, and three hydrogen atoms have been replaced by organic combining groups. Thus methylphosphine (CH₃PH₂) is a primary phosphine, in which the methyl group (CH₃) takes the place of one of the hydrogen atoms of phosphine itself. The metal salts are called phosphides, and the protonated forms (compounds to which a hydrogen ion has been added) are called phosphonium compounds. The organic derivatives of phosphine are usually made by substitution reactions using the readily available phosphorus trichloride (PCl₃).