Scholarship on the hydrosocial cycle has tended to overlook the atmospheric phase of the cycle. This paper identifies and conceptualises a politics of evaporation in Australia’s Murray-Darling Basin. Evaporation is not a neutral hydrological concept to be understood, measured or acted on without an appreciation of the networks in which it originates, the geo-political circumstances that continue to shape its circulation, and its socio-spatial effects. The politics of evaporation is conceptualised here as a process of hydrosocial territorialisation in which atmospheric water came to be known as a force acting within a balanced hydrologic cycle, and ‘atmospheric territory’ was created. The scientific origins of evaporation show (i) how modernist hydrologic technologies and conventions that relied on containment and territorialisation to account for and control water led to the negative depiction of evaporation as a loss, and (ii) the historical depth of processes of abstraction and commensuration that are so influential in today’s regimes of water accounting and marketisation. The politics of evaporation is identified empirically in the controversy surrounding the management of the Menindee Lakes and the lower Darling River in New South Wales, where efforts to ‘save’ water according to the logic of efficiency have enrolled atmospheric water into a Basin-wide program to redistribute surface water. The lens of evaporation theorises a neglected aspect of the materiality of water that is particularly important to the dry, hot parts of the world. It challenges us to rethink the ‘cycle’ as well as the ‘hydro’, while providing further evidence of the value of thinking about territory in a material register as volumetric and not areal.