We have applied an electrochemical capacitance/voltage method to determine doping profiles and material quality of n-type AlxGa1-xAs:Si heterostructures with x ranging from 0.24 to 0.86 in a single measurement step. It was found that the etching rate of the high content Al samples is low and the resulting surfaces are rough, independent of the etchant used. Though surface roughing is associated with an inherent property of AlxGa1-xAs, the doping depth profile can be determined in an accurate way. On the basis of secondary ion mass spectroscopy, Hall effect, and electrochemical capacitance/voltage measurements the total donor concentration could be estimated. (C) 2003 The Electrochemical Society.