CHEMICAL depletion of ozone in the lower stratosphere decreases the direct radiative forcing from greenhouse gases in the atmosphere(1). Here we show that ozone depletion may also exert an indirect effect on radiative forcing via its effect on the oxidation state of the atmosphere. Hydroxyl (OH) radicals in the troposphere are produced by photodissociation of tropospheric ozone in the presence of water vapour, and this process is enhanced if the absorption of ultraviolet radiation by the overlying stratospheric ozone column decreases. As OH oxidizes SO2 to sulphuric acid, which then forms cloud condensation nuclei(2), variations in tropospheric OH concentration can influence cloud albedo. We use a global two-dimensional model forced by observed changes in stratospheric ozone to calculate the consequent changes in production of sulphuric acid over the past decade, and thus to estimate the effect on cloud albedo. We find that this indirect effect of ozone depletion may decrease radiative forcing (via increased cloud reflectivity) by at least as much as the direct effect.