Elastic and inelastic tunneling processes are investigated in GaAs–AlAs–GaAs double heterojunctions grown in the [100] direction by metal–organic chemical vapor deposition (MOCVD). The AlAs quantum barriers in the heterostructures studied are doped p-type with Mg. Theoretical calculations of tunneling currents are performed and compared with experimental I–V data. It is found that for structures with thin AlAs barriers, the dominant current transport mechanism at low temperatures is tunneling through the AlAs band gap at both the Γ and X points. This is consistent with inelastic processes observable in first (dI/dV) and second (d2I/dV2) derivative spectra obtained with modulation techniques. A simple model, developed for calculating impurity-assisted tunneling currents, shows that the role of barrier impurities becomes more important as the barrier is grown thicker. Implications of some of these results for resonant tunneling heterostructures consisting of two AlAs quantum barriers separated by a GaAs quantum well are discussed. Experimental second derivative spectra showing reproducible features are also presented for these double barrier structures.