1. A method of correcting erroneous intensity measurements caused by defective pixels of the detector of a single-crystal X-ray diffraction system during the collection of diffraction images from a plurality of different scan angles for a crystal mounted in the system, the method comprising the steps of: a) predicting reflection positions on the detector from the diffraction images using an orientation matrix established for the crystal; b) constructing a three-dimensional observed profile for each reflection, said observed profile being indicative of pixel intensity relative to scan angle for detector pixels falling within a predicted reflection position as determined in step (a); c) constructing a model profile as an average of the normalized profile data from a plurality of observed profiles; d) providing a defective pixel list for the system indicative of the location of defective pixels in the detector; and e) for each reflection under examination, comparing the defective pixel list with the observed profile for that reflection to determine components of that observed profile that are affected by a defective pixel, and replacing said components with corresponding components from the model profile.