The influence of surface states on the properties of a Al0.2Ga0.8As/In0.18Ga0.82As modulation-doped heterostructure has been measured experimentally. By comparing gated and ungated structures, it is shown that the recess etch of a thin doped cap results in the drastic reduction of both the charge density and mobility in the two-dimensional electron gas, with the charge density and mobility at low temperatures dropping by a factor of 2.1 and 4.5, respectively. By illuminating the sample, there is a small change in the charge density and mobility which can be attributed to the activation of acceptor type deep levels in the AlGaAs. Furthermore, by applying a positive gate bias, most of the mobility is recovered in both the dark and illuminated cases. The charge control law at low temperatures for a gated Hall bar shows nonlinearities that are attributed to interface states and not due to parallel conduction, DX centers, or deep level states in the bulk of the AlGaAs. These results show that the change in the nature and occupation of the states at the surface and the Al0.2Ga0.8As/In0.18Ga0.82As interface can have a direct impact on the properties of a heterojunction 40 nm away.