Photoreflectance studies were carried out on AlxGa1-xN/GaN heterostructures confining high-mobility polarization-induced two-dimensional electron gases. By analyzing the Franz-Keldysh oscillations for samples with (Al)Ga- and N-face polarity, we obtained values for the surface electric field up to F= 380 kV cm(-1) at room temperature. Taking into account spontaneous and piezoelectric polarization, the density of charged surface states and the bare surface potential are estimated. The results unambiguously prove the presence of donor- and acceptor-like surface states for samples with (Al)Ga- and N-face polarity, respectively. A change of the electric field was observed upon the exposure of the surface to a polar liquid, demonstrating the applicability of these structures for chemical sensors. (C) 2003 Elsevier B.V. All rights reserved.