In order to increase productivity of microfluidic contactors, a large number of units operating in parallel is required. For operations involving heat transfer, mass transfer and reactions, fluid velocity and residence time in the microchannels play a crucial role in system performance. Therefore it is important to design microstructures able to guarantee satisfactory flow equipartition within the microchannels. In this work, flow distribution in microstructured plates of varying geometries was investigated. CFD calculations showed that two-dimensional simulations can be misleading in assessing flow maldistribution. Three-dimensional models revealed the presence of a critical value of Reynolds number at which a transition occurs from a flow regime fully determined by viscous forces (where fluid distribution is independent of flow rate) to a regime where inertial effects start to affect fluid distribution. It was found that flow distribution improves by including fins in the plate to guide the flow, increasing the length of the fins and decreasing the plate width. Increasing the length of flow distribution chambers also improves flow uniformity but a limit is reached for the geometries considered, beyond which no further improvement is obtained. Furthermore, modification of the flow distribution chambers and positioning of the inlet and outlet in line with the microchannels can improve flow uniformity.