Two experimental techniques are used in the investigation of cubic boron nitride (CBN) and polycrystalline diamond (PCD) as cutting tool materials for titanium alloy workpieces, in comparison with the currently used coated tungsten carbide specifications. One employs a quasi-static contact method to establish the temperature above which marked adhesion and welding occurs between the tool and the workpiece materials. After separation, the mode of failure of the welded junctions is studied to establish the path of crack propagation. The critical temperatures are shown to be 740, 760 and 900°C for the carbide, PCD and CBN tools, respectively, and in all cases failure of the junctions occurs in the bulk of the tool material. The other method used is the 'quick-stop' technique, under otherwise normal cutting conditions, to study chip formation and tool wear. The predominant wear mechanisms are identified and discussed for each of the tool materials and reasons advanced for observed differences in performance when removing the material from a titanium alloy workpiece. The wear resistance and quality of the machined surface is observed to be consistently better with the ultra-hard materials than with the carbide, and in particular, the PCD tool produces exceptionally good surface finish. In the case of the carbide tool, the rapid removal of the coated layers, leaving the substrate vulnerable to reaction with the workpiece material, is seen as contributing to its relatively poor performance.