The conversion to more environmentally friendly refrigerants is of prime importance for refrigeration and air-conditioning industries. A first option for the substitution of the fluids which use was banned could be natural refrigerants, such as CO2, or HFC mixtures with low global warming and zero ozone depletion impacts. To determine the heat transfer characteristics during flow boiling of CO2 a remarkable amount of literature on flow boiling of CO2 has been published, especially during the last decade. The experiments on this fluid pointed out that, at medium reduced pressure, the heat transfer coefficients trends versus vapor quality are quite different than those commonly encountered with other refrigerants at ordinary temperatures and several interpretations for this were proposed. In this work tests for CO2 and R410A are carried out to study the behaviour of the local heat transfer during evaporation at medium reduced pressure (0.50). The tests were run on a smooth, horizontal, stainless steel tube with an inner diameter equal to 6.00 mm, varying the mass flux between 200 and 350 kg m(-2) s(-1) and the heat flux between 5.0 and 20.0 kW m(-2) over the whole range of vapor qualities. It was verified by experiments that for R410A at a medium reduced pressure the peripheral local heat transfer coefficient trends, as the two-phase flow structures and flow regime transitions, are similar to those for CO2, at the same reduced pressure. (C) 2011 Elsevier Ltd. All rights reserved.