Abstract: When the gas flowing out of the bubble in a gas–solid fluidized bed is recirculated to the bottom and then re-enters the bubble, a clouded bubble is formed. Or else, the gas directly flows into the emulsion phase, forming the cloudless bubble. Owing to different flow patterns between clouded bubble and cloudless bubble, there are different mechanisms for the bubble-to-emulsion mass transfer process. In this work, mass transfer behaviors of an isolated clouded bubble are studied by a coupled computational fluid dynamics-discrete element method (CFD-DEM). The mass exchange contributions by convection and diffusion are evaluated. The results reveal that the diffusion-induced mass transfer is more significant for the clouded bubble than that for the cloudless bubble. The predicted mass transfer coefficient considering the local gas concentration in the emulsion phase shows a good agreement with the correlation prediction by Kunii and Levenspiel (1991). Moreover, the impact of the non-isothermal condition is also explored. When the hot tracer gas is injected into the cold bed, a reverse convective mass transfer is presented at the start stage after the gas injection owing to a strong driving force of gas suction in the bubble wake. Whereas for the tracer gas injection at the room temperature into the bed with pre-heated particles, the mass transfer by diffusion is enhanced and the reverse mass transfer process disappears.