The effective diffusion coefficients of novel, environmentally friendly, hydrofluorocarbon (HFC) blowing agents for polyurethane foam production were measured. The method was based on gas chromatography and infrared spectroscopy measurements. The accuracy and reliability of the procedure developed were confirmed by comparisons of experimental and the predicted thermal aging curves. The slope of the aging curves (i.e., the change in thermal conductivity with time) depended only on the effective diffusion coefficients. The effective diffusion coefficients of the new HFC blowing agents were close to those of conventional hydrochlorofluorocarbon (HCFC) blowing agents, and the aging behavior of foams produced from HFC or HCFC blowing agents were similar. (C) 2004 Society of Plastics Engineers.