One of the main applications of microscale flow is miniature, high-efficiency heat transfer. The most simple and immediate solution to the problem of concentrated heat exchange is the use of small diameter channels with single-phase water flow, but there is a lack of publicised knowledge about the heat transfer performance in these conditions. In this article, an experimental investigation is reported to accurately characterize the diabatic behaviour of single-phase laminar flow in circular microducts, ranging in diameter from 528 down to 120 mu m. The experiments on a capillary of 50 mu m ID proved upon analysis to be unaccountable due to the intrinsic error contained in the set-up, which is tied to the large inertia of the pipe wall, etc. in proportion to the small passage of flow at this diameter. The vacuum environment in which experiments were carried out ensured a test section free of convective losses, so that measurements are provided as precise as possible within the geometry of the microchannel. The possible occurrence of scaling effects such as axial conduction in the walls, viscous heating of the fluid and thermal entrance length effects was studied and criteria were established which have been validated by the measurements. Results show a decrease of Nusselt number with decreasing diameter, an axial dependence that is linked to thermal entrance effects and a dependence of the Nusselt number also on Reynolds number, whence the large conductive losses from the test section can be deduced, not necessarily restricted to axial redistribution of the heat flux in the wall only. (c) 2006 Elsevier Ltd. All rights reserved.