The cocrystallization of nickel ammonium sulfate with magnesium ammonium sulfate has been studied at low concentrations of the nickel salt, using radiotracer63Ni. The applicability of the equations developed byBerthelot andNernst, Doerner andHoskins, andAbu Elamayem is discussed. It is concluded that only the logarithmic equation of Doerner-Hoskins holds good for this picromerite system. This reveals that during crystallization the crystals that separate are in momentary equilibrium with the mother liquor and hence the composition of the crystals separating is not homogeneous. Crystallization at higher temperature yields purer crystals of the magnesium salt.