A method is proposed to estimate the amount of λ/2 contamination inherent in area-detector [charge-coupled device (CCD) or image plate] data using Mo radiation. The intensity increase due to the Mo λ/2 contribution to Ihkl has been determined by measurement of the intensity for reflections where h, k or l is half-integral, i.e. reflections where there is no contribution from λ, for three crystals using two different CCD diffractometers. This information is present in all data sets obtained with area detectors but is usually ignored. The correction thus determined has been applied to nine data sets. The improvement to the data, as measured from the least-squares refinement, is shown to be insignificant for routine data sets, even for the weak low-angle reflections. Some reduction in the number of `observed' systematic absences is noted, thus improving space-group assignments. In addition, for strongly diffracting crystals, incorrect unit cells may be obtained owing to the presence of strong λ/2 reflections at the reciprocal-lattice nodes.