Sorption data are presented for N-2, CH4, CO2 and their mixtures on several bituminous coals. The data are used to interpret the application of the Langmuir sorption isotherm to coals. The Langmuir constants show variations consistent with sorbate molecular properties and changes in coal rank; differences in limiting sorption values amongst sorbates are interpreted with the Gurvitsch rule. Deviations from the Lewis rule are used to infer the usefulness of the mixture formulation of the Langmuir isotherm.