The experimental technique and underlying principles of a thermodynamic method for measuring pump efficiency are described. The technique has been successfully applied to a small process pump whose generated head is one order of magnitude lower than the minimum value stipulated by the current British Standard. Agreement with conventional efficiency measurements in water is good, especially when the pump is operating close to peak efficiency. Inaccuracy of measurements in mercury is attributed to thermal losses, which could be overcome. This technique is particularly suitable for turbomachinery running in liquid metals, or other fluids of low specific heat.