Electrical discharge machining (EDM) is one of the most widely used non-conventional machining processes for the production of moulds/dies, cutting tools and aero-engine parts, such as turbine/compressor blades. The paper details experimental research on the surface alloying/modification of chromium martensitic hot-work tool steel components (AISI H13, 55HRC) during EDM die-sinking operations using powder metallurgy (PM) tool electrodes, as a means of achieving enhanced workpiece wear resistance without resorting to a subsequent coating operation. Tool electrode performance of partially sintered WC/Co electrodes operating in a common hydrocarbon oil dielectric was assessed and subsequently compared with that of conventional electrode materials, such as Cu and Graphite. Surface/subsurface observations by optical and scanning electron microscopy (SEM) showed a recast solidified layer of ~ 8 µm when using WC/Co electrodes. Performed XRD and SEM-EDX analysis indicated that WC and Co contained in the PM tool electrodes, together with C decomposed from the hydrocarbon oil during sparking, were transferred and alloyed to the steel substrate surfaces. EDM surface alloyed layers were hardened over 1200 HK0.025. This hardening is related both to the formation of tungsten carbides with different stoichiometries and to the non-equilibrium microstructure evolution. Thickness of the hardened zone was shown to be dependent on EDM operating parameters, in particular peak-current (A) and pulse on-time (s).