Abstract: In this paper the isothermal adsorption processes of binary and ternary mixtures of benzene, toluene and m-xylol are investigated in a fixed bed of activated carbon of different column heights and under various flow velocities. The model established is solved by the orthogonal collocation method and the fourth order Runge-Kutta method. The results show that the orthogonal collocation method has the advantages of less handling before computation and shorter computing time. A good agreement between the computation results and the experimental results is obtained.