In this paper, the role of pore microstructures on the susceptibility of MAX-phase composites to thermal dissociation at 1300-1800 °C in high vacuum has been studied using in-situ neutrondiffraction. Positive activation energies were determined for the decomposition of MAX phases except for Ti 3AlC2 where negative activation energy of 71.9 kJ mol -1 was obtained due to formation of fine pores on TiCx. An Avrami exponent (n) of < 1.0 was determined, indicative of the highly restricted diffusion of Al or Si between the channels of M6X octohedra. The pore microstructure of decomposed MAX phase has been shown to play a vital role in the kinetics of decomposition with coarse-pores facilitating the decomposition process but the fine-pores hindering it. Copyright © (2012) by Asian-Australasian Association for Composite Materials (AACM).