Abstract: The invention relates to inflatable borehole plug assemblies wherein the time required for their inflation may be controlled. In particular, the invention enables inflatable borehole plug assemblies to be lowered to a preselected depth in a borehole before inflation occurs. The inflatable borehole plug assemblies of the invention include a sealed gas-tight inflatable container containing a pressurised vessel having a pressure release valve. The pressurised vessel contains an inflating substance and a time delay liquid, and is adapted so that on operation of the pressure release valve: (i) the time delay liquid is discharged from the pressurised vessel, thereby causing a time delay between the operation of the pressure release valve and discharge of the inflating substance from the pressurised vessel, and (ii) after the time delay liquid has been discharged the inflating substance is discharged, thereby inflating the inflatable container to form a borehole plug.