One of the most important process parameters in making compound products is the expansion of the bonding surface. Bonding is not obtained until a critical surface expansion, characteristic of the deformation process, is reached. This paper deals with an experimental investigation of the influence of a superimposed hydrostatic pressure on the critical surface expansion during a forging process. The critical surface expansion appears to decrease with increasing hydrostatic pressure. This may be due to the fact that the close contact between the materials necessary to obtain bonding is created by a micro-extrusion of the surfaces into each other. This may explain why the bond strength achieved by different processes, such as forging and extrusion, is quite different for the same value of the surface expansion.