Abstract The spatial π distribution during the monolayer compression process was investigated for several aliphatic LB molecules. For stearic acid and cadmium stearate, the prerequisite for the compression process that π is uniform was confirmed to be satisfied. On the contrary, an aluminium stearate monolayer showed a large two-dimensional π distribution, where π was higher near the moving barrier and near the side walls of the trough. The π distributions were also found to depend on the trough size and barrier speed. The flow characteristics of monolayer compression were analysed by taking account of the fluid dynamics of viscous flow. The origin for this π distribution can be ascribed to the inhomogeneous viscous flow decelerated by the existence of the trough boundary.