The crystal chemistry of the Sm3z to Sm2z reduction in tetraborate lattices was investigated. In crystalline SrB4O7 in air it is mainly Sm2z that is incorporated from a melt or glass containing predominantly Sm3z. For the process in air, a reduction and pick-up mechanism is assumed to take place at the crystal/nutrient interface. Stabilization of Sm2z in SrB4O7 at high temperature and in an oxidizing atmosphere seems to be a particular property of the system, because no Sm2z inclusion could be observed along the series MB4O7 (M~Ca, Ba, Cd, Pb), if similar reaction conditions were applied. So far, there is only one other oxide lattice (BaB8O13) known where at high temperatures significant amounts of Sm2z are obtained for reactions in the air. Single crystals of SrB4O7 : Sm2z were grown by the Czochralski method (keff for Sm is 0.5). Optical hole burning experiments for the transition 5D1±7F0 were performed at 80 K. A hole with a width of 0.21 cm21 and a depth of 5.25% was formed for the first time for Sm2z in a borate crystal excited by the beam of a single frequency dye laser. A rather small inhomogeneous linewidth of 0.28 cm21 allowed the burning of a single hole only.