This work represents the first use of proton irradiation to simulate in-core radiation damage in Ti3SiC2 and Ti3AlC2 MAX phases. Irradiation experiments were performed to 0.1 dpa at 350 degrees C, with a damage rate of 4.57 x 10(-6) dpa s(-1). The MAX phases displayed significant dimensional instabilities at the crystal level during irradiation leading to large anisotropic changes in lattice parameter, even at low damage levels. The instabilities were accompanied by a decomposition of the Ti-based MAX phases to their binary constituents, TiC. Experimentally observed changes in lattice parameter have been correlated with density functional theory modelling. The most energetically favourable and/or most difficult to recombine defects considered were an M-A antisite (M-A: A(M)), and carbon Frenkel (V-C: C-i). It is proposed that antisite defects, M-A: A(M), are the main contributor to the observed changes in lattice parameter. The proposed mechanism reported in this work potentially enables to design MAX phase compositions, which do not favour antisite defect accumulation. In addition, comparison between the experimental results and theoretical calculations shows that a greater amount of residual damage remains in Ti3AlC2 when compared to Ti3SiC2 after the same irradiation treatment. (c) 2018 Elsevier B.V. All rights reserved.