In the attempt to gain a broader understanding of the causal relationships behind work-related symptoms of pain in the human shoulder, monitoring of arm position is crucial. Different methods have been used with varying accuracy. A video-based stereometry system, using infra-red light and reflecting markers for motion analysis, has been introduced for measurements in the fields of ergonomics, biomechanics and sports medicine. The purpose of this study is to investigate the sources of error in using this system for posture registration of the upper limb. Measurements are performed on a calibration fixture, on a mechanical model of the upper limb and on a subject with an exoskeleton. Particular, attention is given to inconsistencies and relative errors due to the finite geometrical precision with which the markers are positioned in the calibration fixture and on the studied objects, the limited capability to align the objects relative to the coordinate system of the calibration fixture and the errors connected to angular measurements using protractors etc. It is concluded that the system makes a valuable addition to existing instruments for non-contact posture measurement, and produces position data with an adequate accuracy in normal handling.