Using an ab initio pseudopotential approach, we have investigated the electronic structure of ideal Al/GaAs(100) and Al/Ga1-xAlxAs(100) junctions, and the change of the corresponding Schottky barrier height versus the alloy composition x and in the presence of ultrathin group-IV atom interlayers. We find large changes in the Schottky barrier height which agree well with the experimental data. In order to interpret the observed trends we have analyzed the charge density response to chemical substitutions near the junction. This allowed us to extend to metal/semiconductor interfaces a microscopic linear-response theory approach previously employed to interpret band-offset trends at semiconductor heterojunctions.