Three-dimensional X-ray diffraction was applied to characterize the strain/stress evolution in individual grains of an Al-0.3 wt% Mn polycrystal deformed in situ at a synchrotron source. Methodological aspects concerning the calibration of the geometrical setup and the evaluation of the strain/stress tensors are discussed. A two-step calibration method separately treating the detector and the rotation axis allows one to determine the centre-of-mass position and crystallographic orientation of grains with standard errors of about 1.5 mm and 0.02 degrees, respectively. Numerical simulations indicate that the error of normal strain components (about 1 x 10(-4)) is mainly caused by calibration errors, while the error of shear components (about 0.5 x 10(-4)) is largely influenced by counting statistics and random spot-centre errors due to detector distortion. The importance of monitoring the beam energy is emphasized.