The separation of hydrogen from carbon monoxide (syngas ratio adjustment) with polymeric membranes was investigated in this work. A polyimide hollow fiber membrane module was used for hydrogen separation. This polymer has shown large permeability and selectivity for hydrogen separation (selectivity of ca. 30). Permeation tests were carried out at different feed conditions. Feed flow rates were varied between 150-300 mL/min, temperature was varied in the range of 20-80 degrees C and feed pressure was varied between 5-9 bar. Mixtures containing 0-50% carbon monoxide were used when carrying out experiments. Measured membrane permeances for hydrogen and carbon monoxide were about 70100 GPU (gas permeation units) and 3-5.5 GPU, respectively. In addition, a mathematical model for simulation of gas separation in hollow fiber membrane modules with all flow patterns (crossflow, countercurrent and cocurrent) was presented. This model can be used for calculation of membrane performance or its required surface area for a specific separation. Experimental results have shown good correlation with simulation results. Plasticization, competitive sorption and concentration polarization effect of carbon monoxide on membrane performance is shown with experimental results. This effect reduced hydrogen permeances in mixed gas experiments.