The vaporisation rates of volatile fission product compounds are critical parameters for modelling aerosol formation following a severe nuclear accident. The vaporisation of CsOH and CsI was studied in a pure steam atmosphere at ambient pressure (85-89 kPa) by increasing the temperature of the flow furnace up to 1000°C. For this purpose, samples were doped with a small amount of radioactive tracer. The vaporisation rate was then determined from the decrease in sample activity with time, using a germanium gamma detector placed outside the furnace. Calculated vaporisation rates obtained by solving complete velocity, temperature and vapour concentration profiles surrounding the sample with FLUENT CFD-software, were in reasonable agreement with the data. A simple engineering calculation agrees almost perfectly with the FLUENT results, if a constant value, Sh≈8, for the Sherwood number is used.