Better reproducibility, greater accuracy, and savings of time and reagents can be realized by omitting the usual procedure of setting up a linear concentration gradient when using a ``well stirred'' diaphragm type diffusion cell. The equation which describes the cell characteristics starting with pure solvent in the diaphragm and in one compartment and the concentrated solution in the other compartment is log[(1−&lgr;/6)&Dgr;Ci/&Dgr;Cf] = &bgr;Dt(1−&lgr;/6).