Heat transfer to the wall of a small pressure vessel during filling with some different kinds of gas was investigated experimentally. The vessel was orientated vertically with the inlet at the top. The space-averaged Nusselt number for the curved wall was found to be a function of both the Reynolds and Rayleigh numbers indicating a mixed convection heat transfer situation. A correlation is proposed for the heat transfer coefficient during charging of the vessel. For the six positions where measurements were taken, the local heat transfer coefficient typically did not differ from the space-averaged value by more than about 30 percent. Measurements were also taken during discharging to atmospheric pressure. For discharging, some of the data was found to agree with a correlation for natural convection in cylindrical geometry. Local Nusselt numbers for discharging tended to be higher towards the bottom of the vessel.