Abstract The water jet erosion behaviour of Ti-Ni alloys has been studied. The variation in erosion resistance with chemical composition of alloys containing 54–60 wt.% nickel is described and discussed in terms of the microstructure of these alloys. Erosion tests were conducted using a high velocity (370 m s −1) water jet for 2–60 min. The erosion resistance of the Ti-Ni alloys was found to be strongly dependent on chemical composition and microstructure but not on hardness. The erosion resistance appeared to be at its best in alloys with a nickel content of approximately 55–56.5 wt.%; there was a sharp reduction in resistance when the nickel content of the alloys was reduced and a gradual reduction along a series with increasing nickel content. The differences in erosion resistance were due to differences in the incubation time for the erosion. The weaker erosion resistance observed at the low-nickel end of the series of alloys was a result of continuous brittle Ti 2Ni phase precipitation. The somewhat reduced erosion resistance in the nickel-rich alloys may be due to localization of the strain energy with precipitation of a hard secondary phase, presumably of TiNi 3. The extraordinarily good erosion resistance of the TiNi phase must be related to the thermoelasticity of its martensite, which can absorb energy elastically.