Internal conversion (often abbreviated IC) is an atomic decay process where an excited nucleus interacts electromagnetically with one of the orbital electrons of an atom. This causes the electron to be emitted (ejected) from the atom. Thus, in internal conversion, a high-energy electron is emitted from the excited atom, but not from the nucleus. For this reason, the high-speed electrons resulting from internal conversion are not called beta particles, since the latter come from beta decay, where they are newly created in the nuclear decay process.

IC is possible whenever gamma decay is possible, except if the atom is fully ionized. In IC, the atomic number does not change, and thus there is no transmutation of one element to another. Also, neutrinos and the weak force are not involved in IC.

Since an electron is lost from the atom, a hole appears in an electron aura which is subsequently filled by other electrons that descend to the empty, yet lower energy level, and in the process emit characteristic X-ray(s), Auger electron(s), or both. The atom thus emits high-energy electrons and X-ray photons, none of which originate in that nucleus. The atom supplies the energy needed to eject the electron, which in turn causes the latter events and the other emissions.

Since primary electrons from IC carry a fixed (large) part of the characteristic decay energy, they have a discrete energy spectrum, rather than the spread (continuous) spectrum characteristic of beta particles. Whereas the energy spectrum of beta particles plots as a broad hump, the energy spectrum of internally converted electrons plots as a single sharp peak (see example below).

In the quantum model of the electron, there is non-zero probability of finding the electron within the nucleus. In internal conversion, the wavefunction of an inner shell electron (usually an s electron) penetrates the nucleus. When this happens, the electron may couple to an excited energy state of the nucleus and take the energy of the nuclear transition directly, without an intermediate gamma ray being first produced. The kinetic energy of the emitted electron is equal to the transition energy in the nucleus, minus the binding energy of the electron to the atom.

Most IC electrons come from the K shell (the 1s state), as these two electrons have the highest probability of being within the nucleus. However, the s states in the L, M, and N shells (i.e., the 2s, 3s, and 4s states) are also able to couple to the nuclear fields and cause IC electron ejections from those shells (called L or M or N internal conversion). Ratios of K-shell to other L, M, or N shell internal conversion probabilities for various nuclides have been prepared.

An amount of energy exceeding the atomic binding energy of the s electron must be supplied to that electron in order to eject it from the atom to result in IC; that is to say, internal conversion cannot happen if the decay energy of the nucleus is less than a certain threshold.

Though s electrons are more likely for IC due to their superior nuclear penetration compared to electrons with greater orbital angular momentum, spectral studies show that p electrons (from shells L and higher) are occasionally ejected in the IC process. There are also a few radionuclides in which the decay energy is not sufficient to convert (eject) a 1s (K shell) electron, and these nuclides, to decay by internal conversion, must decay by ejecting electrons from the L or M or N shells (i.e., by ejecting 2s, 3s, or 4s electrons) as these binding energies are lower.