Abstract The luminescence of the Eu 3+ ion in gypsum, CaSO 4 ·2H 2 O, crystallized from aqueous solutions, is investigated in order to contribute to the unravelling of the precipitation mechanism of gypsum in industrial processes where all kinds of impurities are distributed between the solution and the crystals. Na + -containing solutions show NaEu(S0 4 ) 2 ·H 2 O crystallization next to Eu 3+ incorporated into the gypsum lattice. The Eu 3+ ion in gypsum shows extraordinary luminescence characteristics, caused by a strong linear crystal-field. The quantum efficiency of the luminescence is low due to the strong relaxation in the excited charge-transfer state and the quenching by the water molecules coordinating the Eu 3+ ion in the gypsum lattice. The Eu 3+ ion can be used as a probe to investigate lanthanide impurities in crystallized gypsum.