We have studied the two-dimensional properties of high-mobility electron-hole systems in InAs quantum wells bounded by Ga1-xAlxSb. By changing the alloy composition x, we are able to vary the system from quasi-semimetallic (with unequal electron and hole carrier concentrations) to semiconducting (with only electrons). Furthermore, we have observed a magnetic-field-induced semimetal-to-semi-conductor transition which manifests itself in the magnetotransport properties of samples with both carriers. This transition can be understood by considering the continuity of the Fermi energy across the InAs/Ga1-xAlxSb interface and the interdependence of the electron and hole densities. © 1987.