The length changes during ageing at 320°C of alloys containing 1.61 to 1.80 weight per cent beryllium and 0.22 to 0.29 weight per cent cobalt have been investigated for various geometries, solution treatment temperatures and quench media. Considerable variation in length change has been observed when the alloys are in strip form. This is shown to be associated with plastic deformation during quenching from solution treatment temperature. A quenching procedure has been developed which eliminates this plastic deformation and gives uniform length change. Upper and lower limits of quench range are proposed and computer programs provided to allow of their evaluation for a range of simple geometrical forms.