Membranes of synthetic poly(α-amino acids), namely, poly(γ-methyl L-glutamate) (PMLG), poly(γ-benzyl L-glutamate) (PBLG), poly(L-glutamic acid) (PLGA), poly(L-methionine) (PLM), and poly(Nϵ-carbobenzoxy-L-lysine) (PCLL), were prepared and their permeabilities of oxygen dissolved in water were measured in the 8–50°C temperature range using an oxygen electrode. Permeation curves for the poly(α-amino acid) membranes did not approach steady-state currents because of membrane degradation. To eliminate this, the membranes were laminated between polystyrene membranes; thus, the poly(α-amino acid) membranes came in direct contact with neither cathode surface nor electrolyte solution. No effect of membrane thickness on the permeability was observed. The Arrhenius plots of permeability coefficients for PCLL appear to change slope at about 22°C. This is consistent with the diffusion of oxygen in PCLL through the side-chain regions between helices. Comparisons between the permeability of oxygen dissolved in water and permeability of gaseous oxygen obtained by the high-vacuum method and between the activation energy of permeation of dissolved oxygen and that of gaseous oxygen were made in order to elucidate the effect of water on the oxygen permeation of each polymer. The permeability of the poly(α-amino acid) membranes to dissolved oxygen appears to depend on the properties of the side chains of the polymers.