At low temperatures, typically up to 30 K or even higher, the electrical properties of bulk ZnO samples are nearly always dominated by a conductive near-surface region. Here we show that a single, low-temperature Hall-effect measurement, say at 20 K, and a reasonable assumption regarding the upper limit of the surface compensation ratio, yields a value of surface donor concentration N-D,N-surf accurate to within about a factor two. Examples are given for bulk materials grown by the vapor-phase, melt, and hydrothermal processes. (C) 2007 Elsevier B.V. All rights reserved.