The discussion on renewable sources of synthesis gas and the discussion about flaring associated gas initiated new attraction on Fischer-Tropsch (FT) synthesis with a specific focus on smaller scale due to the distributed nature of the feedstock. Microstructured reactors have often been discussed as solution for process intensification at isothermal conditions and are thus supposed to be suitable for small scale applications in FT synthesis. Catalyst integration into these reactors can be done by packing of particles, but hydrodynamics and heat transfer in the case of gas-liquid flow under FT conditions with unusual ratios of particle diameter to channel or slit width are not accessible by calculation methods without many assumptions and uncertainties. An analysis of the product distribution in two different microstruc-tured reactors with different slit width at otherwise constant conditions revealed that slight temperature gradients in the catalyst bed over the slit width may be the reason for ca. 3% lower chain growth probability at channel width 1.5 mm compared to 0.8 mm. By applying predictions from kinetic parameters from literature, the estimated peak Delta T was 6 K in the larger microstructure. When this peak Delta T was used to calculate the change in methane selectivity and conversion, however, the methane selectivity increase with slit width was over-predicted and a conversion change was assumed, which did not occur in the experiments. Nevertheless, the experiments and the comparison with calculated chain growth probability are indicators for isothermal conditions in channel widths <1 mm. (C) 2016 Elsevier B.V. All rights reserved.