The behavior of a vapor chamber is strongly coupled to the thermophysical properties of the working fluid within. It is well known that these properties limit the maximum power (heat load) at which a vapor chamber can operate, due to incidence of the capillary limit. At this limit, the available capillary pressure generated within the wick structure balances the total pressure drop incurred along the path of fluid flow within the wick. A common figure of merit prioritizes working fluids that maximize this capillary-limited operating power. The current work explores working fluid selection for ultra-thin vapor chambers based on a thermal performance objective, rather than for maximized power dissipation capability. A working fluid is sought in this case that provides the minimal thermal resistance while ensuring a capillary limit is not reached at the target operating power. A resistance-network-based model is used to develop a simple analytical relationship for the vapor chamber thermal resistance as a function of the working fluid properties, operating power, and geometry. At small thicknesses, the thermal resistance of vapor chambers becomes governed by the saturation temperature gradient in the vapor core, which is dependent on the thermophysical properties of the working fluid. To satisfy the performance objective, it is shown that the choice of working fluid cannot be based on a single figure of merit containing only fluid properties. Instead, the functional relationship for thermal resistance must be analyzed taking into account all operating and geometric parameters, in addition to the thermophysical fluid properties. Such an approach for choosing the working fluid is developed and demonstrated. (C) 2016 Elsevier Ltd. All rights reserved.