As mechanical structures, robot manipulators deform when they are loaded, and any deformation causes the gripper to deviate from the desired position. This effect is one of the errors that manipulators exibit. The teach-in mode, used for programming today's robot manipulators, disguises deformation errors, and so deformation errors do not crucially affect the performance of the manupulator. However, when manipulators come to be programmed in off-line mode in the near future, deformation errors can be decisive for manipulator performance. In this paper, the structural response of a robot manupulator to load is investigated. Since results could provide a basis for the further analysis of structural components, holographic interferometry has been chosen as the method of investigation. A ruby laser with a 20 nsec pulse duration proved to be suitable for the application and hundreds of sandwich hologram pairs have been made. The results have disclosed the following facts: - Joints are the weakest parts of the structure. - There was a high degree of deformation of some components. The first result enabled the development of a mathematical expression for describing the deformation in relation to mechanical compliance. This expression is suitable for use in compensational measures. As regards to the second result, one of the highly deformed components was reinvestigated by means of finite element computations and, with minor mechanical modifications, a better design was obtained.