The S process (S for slow) is a neutron capture process in the decay of radioactive elements that occurs in lower neutron density, lower temperature conditions. This process tends to produce stable isotopes by moving along the valley of beta stability. Contrast with P and R-processes.

The s process is believed to occur in stars larger than Earth's sun, most notably red giant stars. In contrast to the r-process which is believed to occur over time scales of seconds, the s-process is believed to occur over time scales of thousands of years. The extent to which the s-process moves up the elements on the periodic table is essentially determined by the degree to which the star in question produces iron, which is the "starting material" for this neutron capture - gamma-ray emission - beta decay method of synthesizing new elements. This is why the largest stars with the longest lifetimes are the most likely candidates for significant elemental production via the s-process.

The s-process is often mathematically treated using the so-called local approximation, which gives a theoretical model of elemental abundances based on the assumption of constant neutron flux in a star, so that the ratio of abundances is inversely proportional to the ratio of neutron-capture cross-sections for different isotopes.

Because of the relatively low neutron fluxes expected to occur during the s-process (on the order of 10⁵ to 10¹¹ neutrons per cm² per second), this process does not have the ability to produce any of the heavy radioactive isotopes such as Thorium or Uranium. The cycle that terminates the s-process is:

²⁰⁹Bi + n⁰ --> ²¹⁰Bi + γ

²¹⁰Bi --> ²¹⁰Po + ß^-

²¹⁰Po --> ²⁰⁶Pb + α

Pb-206 then captures three neutrons, producing Pb-209 , which decays to Bi-209 by beta decay, restarting the cycle.