Controlled flow-rate gas injection experiments have been performed onpre-compacted samples of KBS-3 specification M×801 bufferbentonite using helium as a safe replacement for hydrogen. By simultaneouslyapplying a confining pressure and backpressure, specimens wereisotropically-consolidated and fully water-saturated under pre-determinedeffective stress conditions, before injecting gas using a syringe pump.Ingoing and outgoing gas fluxes were monitored. All tests exhibited aconspicuous threshold pressure for breakthrough, somewhat larger than thesum of the swelling pressure and the backpressure. All tests showed apost-peak negative transient leading to steady-state gas flow. Using astepped history of flow rate, the flow law was shown to be nonlinear. Withthe injection pump stationary (i.e. zero applied flow rate), gas pressuredeclined with time to a finite value. When gas flow was reestablished, thethreshold value for gas breakthrough was found to be significantly lowerthan in virgin clay. There is strong evidence to suggest that the capillarypressure for the penetration of interparticle pore space of buffer bentoniteis of such a magnitude that normal two-phase flow is impossible. Gas entryand breakthrough is therefore accompanied by the development of microcrackswhich propagate through the clay from gas source to sink. The experimentssuggest that these pathways open under high gas pressure conditions andpartially close if gas pressure falls, providing a possible explanation ofthe nonlinearity of the flow law.