We report simultaneous magnetotransport and surface acoustic-wave attenuation measurements performed at 1.25 K on a two-dimensional electron gas (2-DEG) GaAs–AlGaAs heterojunction sample. The carrier density was varied by means of the persistent photoconductivity effect. A comparison of the conductivity deduced from the two types of measurements is used to obtain information about the homogeneity of the 2-DEG sample. For this specific sample a simple model shows that there exists a 20% variation in the carrier density at some intermediate carrier densities, while prior to illumination and after saturation with light the spread in carrier density in the sample was only a few percent.