Spontaneous and piezoelectric polarizations, P-SP and P-PE, in the c-axis direction of Zn1-xMgxO/ZnO heterostructures are estimated by analyzing the density of the two-dimensional electron gas (2DEG) accumulated at the interface. The experimental data on 2DEGs indicated that positive sheet charges responsible for the 2DEG accumulation were induced at the interface by the polarization with a proportional increase to the x in Zn1-xMgxO barrier layer. For the samples grown on a-plane sapphire substrates by molecular beam epitaxy (MBE), however, both of the P-SP and P-PE in a thin ZnO cap layer on thick Zn1-xMgxO buffer layer were found to be negative due to the tensile-strained pseudomorphic growth in O polarity. Since negative polarization induces negative sheet charges at the Zn1-xMgxO/ZnO interface, this result should indicate the larger contribution of the P-SP discontinuity at Zn1-xMgxO/ZnO interface than that of the PPE in ZnO cap layer, deducing the x-depeDdent P-SP in Zn1-xMgxO to be -(0.057+0.066x)C/m(2). This interpretation is supported by the theoretical estimation of the P-SP at Zn0.5Mg0.5O using a first-principals calculation. (c) 2006 Elsevier B.V. All rights reserved.