Fuel cells directly fed by liquid methanol represent a class of suitable devices supplying small portable power applications. However some issues must be properly addressed and resolved, before considering them a market attractive technology. The presence of gaseous CO2 generated in the anode channels is the main issue as it can hinder the free volume of the Gas Diffusion Layer (GDL) reducing the methanol flux through the porous media towards the catalyst layer. Here the influence of the gas phase on the cell performance is investigated as well as the relationship with operating parameters, such as air flow rate, methanol-water flow rate and current density. We noticed that higher anodic flow rates can determine energy waste (the utilization factor of the cell can become very low). On the other hand, lower flow rates usually result in generation of gas slugs remaining attached over the GDL surface or slowly moving in the channels. This phenomenon reduces the residence time of the liquid phase and affects the mass transport of methanol towards the Catalyst Layer (CL). The V/I curve, for flow rates lower than 3.4 ml min(-1) cm(-2), results to be lower at high current density, where the main cause of losses is the concentration polarization. The study was performed by micro-PIV and it helped to verify the effective temporary clogging of the anode channel when working in controlled conditions. The characteristics of the CO2 bubbles in the two-phase flow at low flow rate regimes in a square channel were investigated and the mapping of different bubbly flows was carried out. (C) 2016 Elsevier Ltd. All rights reserved.