The rate of penetration and the solubility of H, O, N, NH3, H2O, HCl gas, CO2, formic, acetic, chloracetic, dichloracetic acid, glycerol, phenol and mercury bichloride in dry collodion membranes have been measured. The rate of penetration of H and CO2 is the same whether the membrane and gas are dry or whether the membrane is immersed in water. The solubility of CO2, acetic acid, phenol and water in collodion is completely reversible and is proportional to the concentration (or vapor pressure) in low concentrations and independent of the surface of the collodion. The size of the pores has been calculated from the vapor pressure of water in the collodion and from the rate of flow of water through the membrane. The results do not agree and are not consistent with the observed rates of penetration. The relative rates of penetration of the gases bear no relation to the density of the gas. When the results are corrected for the solubility of the substances in the collodion and expressed as the diffusion coefficient in collodion they show that the diffusion coefficient increases rapidly as the molecular weight decreases.