Torsional vibration measurements on rotating machinery under working conditions have been limited. This is particularly the case for measurements made under controlled excitation conditions. In previous papers, the authors have presented experimental results obtained for a back-to-back gearbox rig with a controlled excitation. This paper is concerned with methods of extracting the torsional characteristics of a single gearbox from the measurements made on the complete test rig that included a servo-motor and a second gearbox. (c) 2004 Elsevier Ltd. All rights reserved.