Abstract An ordinary soap bubble immersed in a pure carbon-dioxide atmosphere, created e.g. by sublimating chunks of dry ice (solid CO2), will expand within a few minutes to a volume exceeding the original one with a factor of almost ten. Considering that the bubble simultaneously experiences a cooling of ∼30∘C one would perhaps expect a tiny shrinkage instead. The physics behind the phenomenon, i.e. diffusion of highly soluble CO2 through the soap film caused by the difference in the partial pressure of CO2 between the interior and the exterior of the bubble, combined with the expansion of the active area of the bubble, is discussed. Actual data obtained from a video of an expanding bubble is combined with the simulation of the process, resulting in a fairly good agreement. The reverse experiment of a bubble filled with CO2 under ambient conditions leads to a fast shrinkage of the bubble. Finally, a bubble filled with helium placed in a CO2 atmosphere expands initially, reaches a maximum volume and then begins to shrink. Also this case was modeled semi-empirically. The experiments presented are suitable for upper-secondary-school pupils. Understanding the theoretical description requires first year university studies of physics or chemistry.