Ultra-high temperature geothermal wells (>450 °C) have a large potential for increased energy yield as compared to conventional high-temperature geothermal wells (200-300 °C), but several research challenges must be resolved before robust operation in this temperature range can be achieved. In this study, yield- and tensile strength data for several relevant carbon steels and corrosion resistant alloys are generated as a step on the way to enable design of collapse- and tensile capacity for geothermal casings exposed to temperatures up to 500-550 °C. The experiments extend the data set listed in NZS 2403:2015 by providing data for higher temperatures and different material classes. It is found that the carbon steels follow the same near linear decay in strength as the NZS 2403:2015 curves up to 350 °C, and then display a significant drop in tensile strength at higher temperatures, particularly for the lower strength steels. The alloys with high nickel content work harden significantly more than the carbon steels at high temperatures and they tend to retain their strength at temperatures above 350 °C. The tested titanium alloy shows high yield strength and low work-hardening at 500 °C and in contrast to the tested nickel alloys, do not display dynamic strain ageing.