The diffusivity of; hydrogen through aluminium, gold, molybdenum and silver; deuterium through gold, molybdenum and silver; helium through aluminium, gold and silver has been studied by a modified response time method using periodically modulated discharge and ion beam injection at the input surface. Measurements consisted of the response time of pressure, across a metal membrane, in the detection chamber due to fluctuations at the input. The downstream vacuum chamber was continuously pumped. The relationship between the time-lag of the downstream pressure relative to the upstream pressure yielded the diffusion coefficient. The permeability of hydrogen and deuterium through molybdenum and gold were determined from separate time independent measurements of the flux through the membrane. The relationships for the time-lags and steady rate fluxes were modified when account was taken of the possible importance of surface processes in permeation. The modified relations were found necessary when analysing the data of aluminium and gold.