The use of thermodynamic predictions of alloy stability to assist with the alloy design process is widespread. Therefore, it is important to understand how accurate these predictions are currently. To this end, a series of model alloys with systematically varying Mo contents have been designed to be unstable with respect to the precipitation of the topologically close-packed phase, sigma. Increasing the Mo content was found to increase the quantity of sigma phase precipitation and alter the precipitate morphology from a plate-like to a more blocky morphology following the same thermal exposure conditions. Differential Scanning Calorimetry has been used to measure the sigma solvus temperatures of these alloys and the results compared with thermodynamic predictions made using Thermocalc software. It was found that the observed trend of increasing solvus temperature with increasing Mo content is reproduced by the predictions, but that the temperatures are significantly under- predicted. This has implications for how we use these values to influence the design of new alloy compositions.