The size of the channels in a monolith reactor is an important geometric factor which affects gas and liquid (G-L) distribution in monolith channels significantly and therefore axial-mixing behavior. In the present work the axial-mixing behavior of the liquid phase in cocurrent down flow square channel monoliths of different sizes (200, 400, and 600 cells per square inch or cpsi) has been studied by analyzing the residence time distribution (RTD) of the injected tracer pulse. Simultaneous pressure drop measurements were used to estimate the average liquid slug length. The liquid hold-up in all monoliths was found to be in reasonable agreement (within 30%) with that of a Taylor flow in a single capillary. Axial dispersion and liquid slug length were observed to decrease with increase in cpsi. When intermonolith G-L redistribution occurred between stacked monolith pieces, the axial dispersion decreased further. The ratio of gas bubble length to total slug length (beta(G)), the total G-L velocity, and the liquid slug length affected the degree of axial mixing in a monolith with beta(G) having a strong negative impact. The liquid corner flow in the square channels appeared to be the main cause of axial mixing and not a liquid film between a gas bubble and a channel wall. Overall axial mixing in the monolith could also be partly attributed to maldistribution of the G-L phases at the monolith entrance. A dimensionless correlation is proposed to estimate the overall flow Peclet number.