This paper explores the performance characteristics of a split reaction water turbine. The governing equations are derived by using the principles of conservation of mass, momentum and energy for a practical case, which includes consideration of frictional losses. The optimum diameter for a simple reaction turbine is defined and an equation for the optimum diameter is derived. Design and building procedures for a split reaction turbine are described. Using the equation for optimum diameter and assuming a loss factor (k-factor) of 0.05, optimum rotor diameters for different operating heads and rotational speeds are plotted and discussed. Measured performance of a 122 mm diameter split reaction water turbine rotor is presented. The relationship between k-factor and relative velocity for a split reaction turbine model is discussed with reference to experimental data. (C) 2010 Elsevier Ltd. All rights reserved.