The emission and excitation spectra of the Eu3+ luminescence and the energy transfer phenomena in single crystals of Gd1−xEuxAl3B4O12 (0<x?1) have been investigated using time-resolved site-selection techniques. In addition, the luminescence properties of single crystals and powders have been compared. The results show that in these compounds there are three major types of Eu3+ sites. Most Eu3+ ions are on the regular crystallographic sites, but a relatively high percentage of these ions are at nonregular (Al3+ and interstitial) sites. The crystals of EuAl3B4O12 show quenching at lower temperatures than do the powder samples. This behavior can be explained with the help of a model which takes into account diffusion-limited migration within the regular Eu3+ system and quenching by transfer to Mo3+ impurities. These impurities are incorporated into the crystals unintentionally during their growth. The concentration quenching in the single crystals is quite strong and can be explained by means of the percolation concept. In the powder samples, however, the concentration and temperature quenching had less influence due to the relatively low concentration of the quenching centers in these samples. In all samples there was evidence of direct transfer from ’’nonregular’’ Eu3+ ions to ’’regular’’ ions. The critical transfer distance was derived.