Phosphorus (P) has 23 known isotopes with a mass number ranging from 24 to 46. Only one is stable, 31P, making phosphorus a monoisotopic element.

Longer-lived phosphorus radioisotopes are 33P (half-life of 25.34 days) and 32P (14.263 days). All other isotopes have a half-life of less than 2.5 minutes, and most of them less than one second.

Phosphorus 33 is an artificial radioactive element. It is produced with a low yield by the neutron bombardment of phosphorus 31 (stable).

The phosphorus 33 has a radioactive period of 25.3 days. It is a pure β-transmitter.

Phosphorus 33 is used as an alternative to phosphorus 32 in research in molecular biology. Indeed, its longer life time and especially its less energetic β spectrum make its manipulation simpler in the laboratory.

In the medical field, phosphorus 33 has been used in the treatment of arterial stenosis but is no longer indicated at this time.