Montmorillonites, before and after being saturated with Na+, K+, H+, Ca2+ and Al3+ ions, were studied to analyse their O-2, N-2, CO, CH4, C2H2 and CO2 gas retentions at 25degreesC and 0.1 MPa, previously degassed at 100degreesC. The homoionic-montmorillonites were characterized by interlayer spacing measurements and BET surface area. The levels of gas adsorptions for C2H2 (0.184-0.218 mmol g(-1)) and CO2 (0.170-0.208 mmol g(-1)) by original and homoionic montmorillonites were higher than those for N-2 (0.068-0.074 mmol g(-1)), O-2 (0.070-0.076mmol g(-1)), CO (0.070-0.080mmolg(-1)) and CH4 (0.082-0.086 mmol g(-1)). The C2H2 and CO2 gas adsorptions by different cation-exchanged forms of montmorillonite follows the following order: H- > K- > Ca- > Al- > Na-montmorillonite. The ionic radius of the exchangeable cations, the interlayer separation and surface area of the montmorillonite influenced the gas adsorption.