Abstract The object of this paper is (a) to bring to the attention of operating companies the benefits derived from index tests of hydraulic turbines, and especially those of the adjustable-blade propeller or Kaplan type; (b) to enumerate and discuss the presently known methods of determining relative discharge and hence relative efficiency; (c) to present an example of an index test of an adjustable-blade propeller turbine in sufficient detail to provide a guide for the conduct of such a test.