Electron mobility of gadolinium/europium (dibenzoylmethanato)(3)(bathophenanthroline) (Gd/Eu(DBM)(3) bath) was measured by transient electro luminescence (EL) method. Although electron mobility of the two complexes were expected to be same, the value of mobility (1.2 x 10(-4) cm(2)/Vs at electric field of 1 MV/cm) of Eu(DBM)(3) bath complex was bigger than that (8 x 10(-5) cm(2)/VS at electric field of I MV/cm) of Gd(DBM)3 bath complex. It was found to be related to the different luminescent mechanisms of active materials and recombination Zones in the devices. According to this, penetration length of hole injected into electron transport layer of Eu(DBM)(3) bath was estimated. (C) 2006 Elsevier Ltd. All rights reserved.