This paper presents the results of our measurements and detailed simulations using GEANT4 to investigate the performance of LaBr 3 : Ce detectors. In the first part of the paper we present the results of our experimental studies using cylindrical 1 in: Γ 1 in: LaBr 3 : Ce scintillator. We have measured the energy and timing resolutions and also the absolute detection efficiencies, both, total detection and photopeak, using known activity sources. The energy dependence of the resolution has been studied using a variety of low energy gamma rays sources. The measured absolute efficiencies have been compared with the simulated values using GEANT4. We have extended our simulations to reproduce the recent measurements (Favalli et al., 2008) [15] of absolute photopeak efficiencies of a 1:5 in: Γ 1:5 in: cylindrical LaBr 3 : Ce detector up to 5 MeV. The second part of the paper is devoted to a detailed comparative study of the energy dependent absolute efficiencies, both total detection and photopeak, of cylindrical LaBr 3 , NaI(Tl), and BaF 2 scintillators of different sizes for a wide range of gamma rays from 662 keV to 50 MeV. We have also simulated the energy dependent efficiencies for compact assemblies of: (a) LaBr 3 : Ce and NaI(Tl) and (b) LaBr 3 : Ce and BaF 2 in two different configurations. The efficiencies, so simulated, for these configurations, have been compared with the efficiencies of single scintillators of equivalent volumes. The usefulness of such combined detector assemblies using LaBr 3 crystals of available volumes with NaIΓ°TlΓ=BaF 2 in measurements of high-energy gamma rays (5-50 MeV) is discussed.