The effect of target molecule depletion from the supernatant solution is incorporated into a physico-chemical model of hybridisation on oligonucleotide microarrays. Two possible regimes are identified: local depletion, in which depletion by a given probe feature only affects that particular probe, and global depletion, in which all features responding to a given target species are affected. Examples are given of two existing spike-in data sets experiencing measurable effects of target depletion. The first of these, from an experiment by Suzuki et al. using custom built arrays with a broad range of probe lengths and mismatch positions, is verified to exhibit local and not global depletion. The second dataset, the well known Affymetrix HGU133a latin square experiment is shown to be very well explained by a global depletion model. It is shown that microarray calibrations relying on Langmuir isotherm models which ignore depletion effects will significantly underestimate specific target concentrations. It is also shown that a combined analysis of perfect match and mismatch probe signals in terms of a simple graphical summary, namely the hook curve method, can discriminate between cases of local and global depletion.