The thermal management system of a fuel cell provides the core heating and cooling requirements to components such as the stack, reactant gases and heat recovery for cogeneration. A microencapsulated phase change material (MCPCM) slurry is proposed as a heat transfer medium in this circuit to provide additional thermal capacity and increased convective heat transfer from the latent heat associated with the solid-liquid phase change. This will enable smaller volumetric flow rate and reduced parasitic loading as a result of smaller pumping requirements, thus improving system electrical efficiency and thermal performance. The slurry further reduces the impact of temperature fluctuations; reduces the risk of overheating that may result in stack damage, and increases the heat exchanger lifetime from reduced thermal stress fatigue. Previous work with phase change material and slurries have been reviewed in parallel with an analysis of a PEM fuel cell, its thermal management system and requirements. Experimental tests have determined the characteristics and quantified thermophysical properties of the MCPCM to be used in the system. This included the distribution of particle size and the morphology under phase change, thermal energy storage and release capacity, and rheology. Further slurry characteristics were analysed that were applicable to the practicality of system performance, such as structural stability. The results of the study demonstrate that encapsulated paraffin wax may be considered to have good potential for use with fuel cell thermal systems.