This paper presents an experimental technique and experimental results of heat transfer measurements in microtubes of 250 and 500 gm diameter. The data obtained with single-phase argon show no physical difference of heat transfer mechanisms between micro- and macrotubes. The enhancement of heat transfer compared to conventional correlations is explained with the increased influence of roughness, and modeled with a new relative roughness parameter. In two-phase flow, nucleate boiling heat transfer coefficients are lower than in macrotubes. An extension of the diameter dependence in the VDI Warmeatlas correlation is proposed, and the conditions for stable flow-boiling are discussed. (c) 2005 Elsevier Ltd. All rights reserved.