A dead‐weight piston gauge was used to determine the bismuth I‐II transition pressure and the volume change at the transition. The transition pressure at 25°C for one sample, believed to contain less than 0.001% impurities, was found to be 25 306 bar, with an estimated uncertainty of 60 bar. With another sample, of substantially less purity and of larger grain size, a transition pressure of 25 500±60 bar was measured. The average relative volume change was 0.035.