We present the first direct measurement of proton energy-loss straggling in Ga1-xAlxAs/GaAs, enabling us to take maximum advantage of the 27Al (p, γ) 28Si nuclear reactions as a powerful nondestructive technique for measuring Al profiles in these structures. Results were obtained using samples produced by molecular beam epitaxy and fabricated to have step-function Al concentration distributions to prescribed depths. The exact straggling width was obtained by a least-square comparison of the experimental spectra with curves calculated using a parameterized straggling distribution. With these results, profiling measurements can now be made giving the Al concentration fall-off at the GaAl/GaAs interface with a spatial depth resolution of about 4% and epilayer thickness determinations to about 2%. We have also observed in one sample a nonabrupt transition at the GaAlAs/GaAs interface, due to differences in substrate surface preparation procedures prior to growth of the GaAlAs layer.