- Neodymium-doped yttrium perovskite YAlO 3 (YAP) is one of the most important solid state laser crystals. Cr 3+ co-doping ions are added to Nd 3+ lasing ions in the YAlO 3 crystal for an improvement of the pumping efficiency. To understand fully the efficiency of these crystals, we must consider the flow between the various ions. Due to the disorder present in the host lattice, one cannot say that all the donors (ions which absorb the pump light, i.e. Cr 3+ in our crystal) or all the acceptors (ions which emit the desired emission, i.e. Nd 3+ in our crystals) are in identical sites. Instead we have multi-sites, with a main site being the crystallographic position where most of the dopant ions fall and sub-sites where a lesser (non-negligible) quantity of ions are located. In this paper we analyze data from Cr,Nd: YAP. We will present a new model which covers the transfer of energy to multi-sites.