The sulphur cycle plays fundamental roles in the chemistry(1-3) and climate(4,5) of Venus. Thermodynamic equilibrium chemistry at the surface of Venus favours the production of carbonyl sulphide(6) and to a lesser extent sulphur dioxide. These gases are transported to the middle atmosphere by the Hadley circulation cell(7,8). Above the cloud top, a sulphur oxidation cycle involves conversion of carbonyl sulphide into sulphur dioxide, which is then transported further upwards. A significant fraction of this sulphur dioxide is subsequently oxidized to sulphur trioxide and eventually reacts with water to form sulphuric acid(3). Because the vapour pressure of sulphuric acid is low, it readily condenses and forms an upper cloud layer at altitudes of 60-70 km, and an upper haze layer above 70 km (ref. 9), which effectively sequesters sulphur oxides from photochemical reactions. Here we present simulations of the fate of sulphuric acid in the Venusian mesosphere based on the Caltech/JPL kinetics model(3,10), but including the photolysis of sulphuric acid. Our model suggests that the mixing ratios of sulphur oxides are at least five times higher above 90 km when the photolysis of sulphuric acid is included. Our results are inconsistent with the previous model results but in agreement with the recent observations using ground-based microwave spectroscopy(11) and by Venus Express(12).