The trend of enhanced functionality and reducing size of mobile devices has led to a rapid increase in power density and a potential thermal bottleneck since thermal limits of components remain unchanged. Active cooling mechanisms are not feasible due to size, weight, and cost constraints. This work explores the feasibility of a passive cooling system based on Phase Change Materials (PCMs) for thermal management of mobile devices. The PCMs stabilize device temperatures due to the latent heat of phase change, thus increasing the operating time of the device before threshold temperatures are exceeded. The primary contribution of this work is the experimental evaluation of key parameters which influence the design of a PCM based thermal management system. In situ measurements with PCMs as a passive thermal management solution demonstrate that a significant extension in the time that the processor can run at full power before the processing power would need to be throttled to prevent damage.