Sulfur (S) exists in solid, liquid and gaseous form and is found in nature in large quantities, attached to other elements in the form of sulfides and sulphates.

Sulfur has 25 known isotopes and a nuclear isomer with a mass number ranging from 26 to 49, of which four are stable: sulfur 32 (S-32) (very predominant), sulfur 33 (S-33), sulfur 34 (S-34) and sulfur 36 (S-36). The preponderance of sulfur 32 is explained by its production from carbon 12 by successive fusion with five nuclei of helium 4 in the alpha reactions preceding the explosions in type II supernova.

The longest half-life radioisotope is sulfur 35 (S-35), with 87.5 days, followed by S-38 (170 minutes) and S-37 (5 minutes). All the other isotopes have a half-life of less than 1 minute, and even for a large part of them less than the second. The isotope with the shortest lifetime is sulfur 49 (S-49), with a half-life of less than 200 nanoseconds. The lighter isotopes than the stable isotopes disintegrate mainly by positron emission (β +) in phosphorus isotopes, the heaviest by β- disintegration in isotopes of chlorine.

Sulfur is an artificial radioisotope: it is obtained by neutron / proton reaction on chlorine or by activation of sulfur.

Sulfur is one of the most widely used riboisotopes in biological and medical research, especially when sulfur methionine is incorporated for protein monitoring.