A three-dimensional computational fluid dynamics (CFD) simulation study of the effects of the flow distribution and the heat losses on the performance of microchannels and microslits reactors for the steam reforming of methanol (SRM) over Pd/ZnO is presented. Several flow distributing headers covering a wide range of the flow diffuser expansion angle (alpha) have been considered. Large values of a lead to flow mal-distribution characterized by jet flow resulting in negative effects on the SRM conversion and hydrogen yield, especially for the microslits at high reaction temperatures and space velocities. Simulations have also evidenced that heat losses constitute a critical issue for microreactors operation, particularly at low space velocities. Heat losses may reach very high values, above 80-90% of the energy supplied to the microreactor, with the consequence that it may be necessary to provide up to 9 times the heat of the SRM reaction to achieve high methanol conversions. (C) 2013 Elsevier B.V. All rights reserved.