[en] Gas-permeable polymeric membranes containing carboxyl groups which are suitable for enzyme immobilization were investigated in order to use them as gas electrode membranes in biosensors. Carboxylated polyurethane (CPU) was synthesized via a reaction between 2,2-bis(hydroxymethyl)propionic acid as a chain extender and prepolymers prepared from polycarprolactone (Mn=2,000) and 4,4'-diphenylmethane diisocynate. It was difficult to prepare membranes from the pure CPU because of its high elasticity and cohesion. However, transparent free-standing membranes were easily prepared from the blend solutions of CPU and carboxylated poly(vinyl chloride) (CPVC) in tetrahydrofuran. Both elasticity and cohesion of the CPU/CPVC membranes were decreased with increasing the content of CPVC. DSC experiment suggests that CPU and CPVC may be well mixed. Permeability coefficients for O₂ and CO₂ (P_O2 and P_CO2) in the membranes increased as the proportion of CPU increased. The addition of dioctyl phthalate (DOP), a plasticizer, significantly enhanced the P_O2 and P_CO2 which were 4.4 and 30 Barrer, respectively, in the CPU/CPVC (80/20 wt/wt) membranes containing 20% of DOP at 25 .deg. C and 100 psi. Thus this type of membranes may have a potential for the use as gas electrode membranes in biosensors