Conventional and truly undrained behaviour of granular soils is compared by direct experimental measurements. The truly undrained behaviour was determined by compensation for membrane penetration induced volume changes, using a feedback controlled water injection system. A more reliable, nondestructive, specimen-specific method is presented to measure the magnitude of membrane penetration, for use in the injection routine. This new method, unlike the existing methods, does not make any assumptions regarding either the constitutive behaviour or isotropy of sands. It is shown that the truly undrained pore pressures predicted by correcting those observed in conventional undrained tests are too large, implying incorrect linkage between membrane penetration induced volume change and volumetric constitutive behaviour of the granular material. The sand which responds in a strain-hardening manner in conventional undrained loading may, in fact, be strain softening (liquefiable) when truly undrained, a matter of serious practical concern.Key words: sand, truly undrained behaviour, liquefaction, membrane penetration.