Uranium mononitride (UN) is an attractive fuel for a range of reactors, including, with adequate protection against reaction with water, for large scale and small modular light water reactors. The study of the speciation of fission products (Fps) in spent UN fuel is required to understand their properties such as phase stability and retention during long term storage and disposal. The present study reviews and applies Hume-Rothery rules to predict this speciation. This law provides an estimate of the way UN may form solid solutions with actinide (An) and fission product mononitrides from Beginning of Life (BoL) to End of Life (EoL). Composition at EoL is estimated for a high burnup fuel (60 MW d kg−1) using the FISPIN fuel inventory code. Many Fps are trivalent (Ln, Y, Zr, Nb) and are expected to recrystallize in face centred cubic (fcc) solid solutions with UN. Other Fps are divalent (Ba, Sr) and monovalent (Cs, Rb) and some elements are non-valent (Mo, Tc, Ru, Rh, Pd) as well as noble gases (Xe, Kr), and halides (I, Br) which may form nano-precipitates. Actinides formed by neutron capture of 238U are all An3+ which are also expected and found in fcc solid solution with UN. The spent fuel is consequently formed of a large fraction of solid soluble nitrides (U, An, Ln, Y, Zr, Nb)N forming a single rather homogeneous phase according to the Hume-Rothery rules. This phase will also follow Vegard's law. Because of the low temperature of the fuel, the Fps are expected to be soluble or nano-dispersed precipitates. Microscopic precipitated phases are not expected.