Conversion and temperature distributions in a tubular reactor with an inner diameter of 1 mm were numerically calculated for a second order exothermic reaction in laminar flow of homogeneous liquid. Based on the resulting radial temperature profiles, the local Nusselt numbers and bulk mean temperatures were determined along the reactor tube. Strong effects of the homogeneous reaction on the heat transfer can be observed in the entrance region of the reactor, where a hot spot emerges. Due to the large radial temperature gradients in vicinity of the reactor wall, heat transfer coefficients are significantly higher compared to a non-reactive system. The consequences of this effect on the design and control of exothermic reactions in reactor/heat exchangers are demonstrated by comparison with a simple one-dimensional plug flow model. In the simplified model, neglecting thermal influence of the exothermic reaction results in a significant underestimation of the required reactor length for defined conversion. Accordingly, numerical simulation of both axial and radial transport in the hot spot region can be essential to precisely predict the bulk temperatures and conversion rates in the reaction mixture, even with the small length scales of milli- and micro-structured reactors. (C) 2015 Elsevier Ltd. All rights reserved.