The dryout for flow boiling carbon dioxide (CO2) in horizontal small diameter tubes is investigated through experiment and theoretical modeling. Tests are conducted in conditions where the saturation temperature is 0, 5, and 10 degreesC, heat flux is 7.2-48.1 kW/m(2) and mass flux is 500-3000 kg/m(2) s. The dryout phenomena of CO2 are similar with those of water in many respects, while the effects of mass flux on dryout show differences among them. The dryout of CO2 is predicted by a theoretical dryout model, which is developed and verified with steam-water data. Two entrainment mechanisms of interface deformation and bubble bursting are considered in the model and dryout is determined when the liquid film thickness is less than the critical liquid film thickness, the criteria film thickness of dryout. The present model well predicts the experimental critical qualities except when mass flux is relatively high, at which the deposition of liquid droplet on the liquid film and the occurrence of dryout patches become very significant. (C) 2003 Elsevier Science Ltd. All rights reserved.