Abstract This paper explores the suggestion by Idso and his colleagues that the response of stomata to saturation vapour pressure deficit may prove to be an artefact of porometry. Re-examination of their analysis reveals several potential sources of error leading to anomalously small ratios of aerodynamic to stomatal resistance both for leaves and for canopies. Arguments are advanced for adopting values of this ratio an order of magnitude larger than those used by Idso. The saturation deficit ( D 0) estimated within the canopies explored by Idso is then less than 1 kPa even when the deficit in ambient air approaches 30 kPa. As Idso's own porometer measurements suggest that stomatal closure in water hyacinth did not occur until D 0 exceeded 1 kPa, his observations of stomatal resistance and radiative surface temperature appear to be fully compatible and his “apparent discrepancy” disappears.