A study of foams as gas-blocking agents in model porous media is reported. Foams are generated by displacing surfactant solution with gas, and tested by meas·uring gas permeability as a function of step-wise increasing differential pressure. The method gives a direct, in-situ measurement of foam gas-blocking ability which is sensitive to differences among surfactants tested. Contrary to some previous reports, no correlation is found between surfactant performance in gas-blockage, tests and in bulk foam tests. It is suggested to employ the gas-blockage method for screening purposes instead of bulk tests in studies directed at obtaining maximum gas blockage of porous media.