There has been significant commercial interest in the use of oscillating foil energy converters (OFECs) to extract renewable energy from tidal streams. The majority of research into OFECs has focused on a foil undergoing a prescribed pitch and heave and does not take into account the complex fluid-structure interaction found in commercially developed elastically supported OFECs. To address this need, an experimental investigation was conducted of a model OFEC undergoing a prescribed pitch with a heave determined by the response of the mechanism to unsteady hydrodynamic forcing on the foil. The aim of the tests was to investigate the influence on power generation and efficiency of key dimensionless parameters; reduced frequency (k), and pitch amplitude (α0). For a fixed dimensionless damping coefficient of C ′ = 29.5 the device achieved an efficiency of 23.8%. This occurred at a high pitch amplitude of α0 = 58◦ and a reduced frequency of k = 0.1 which is in agreement with numerical simulations of OFECs undergoing a prescribed pitch and heave.