Until the relationship between in-vivo and ex-vivo diffusion measures has been established, the clinical relevance of findings using ex-vivo diffusion MRI is unclear and precludes potential studies. Therefore, this study investigated the effects of tissue fixation on basic diffusion measures, and established the relationship of diffusion measures recorded in-vivo and ex-vivo. Basic diffusion measures of postmortem hemispheres were observed over 5 weeks. The relationship between in-vivo and ex-vivo diffusion measures was studied using linear mixed regression. Appreciable changes to diffusion measures were seen early on in fixation, and ex-vivo measurements of FA and RD were linked to their in-vivo measurements.