New data for a relatively large scale thermosiphon reboiler show relations between operating conditions and maximum heat flux limitations of the fIlm boiling, mist flow and instability types. These data are compatible with previous data on a small unit taken by Lee, et al.[3] and indicate that the Lee correlation may be used to estimate larger reboiler maximum heat fluxes over the same ranges of reduced pressures and tube geometries. It is shown that a simplified reduced property relation can predict maximum heat fluxes of all three types. It is postulated but not yet proven that all types may be initiated by similar conditions with the difference in type of limitation being a function of the amount of channel resistance and the reduced pressure.