Let $f:X\to Y$ be a Cohen–Macaulay map of finite type between Noetherian schemes, and $g:Y'\to Y$ a map, with Y′ Noetherian. Let $f':X'\to Y'$ be the base change of f under g and $g':X'\to X$ the base change of g under f. We show that there is a canonical isomorphism $\theta_g^f: {g'}^*\omega_f \simeq \omega_{f'}$, where $\omega_f$ and $\omega_{f'}$ are the relative dualizing sheaves. The map $\theta_g^f$ is easily described when f is proper, and has a more subtle description when f is not. If f is smooth we show that $\theta_g^f$ corresponds to the canonical identification $g'^*\Omega_f^r= \Omega_{f'}^r$ of differential forms, where r is the relative dimension of f. This work is closely related to B. Conrad's work on base change. However, our approach to the problems and our viewpoint are very different from Conrad's: dualizing complexes and their Cousin versions, residual complexes, do not appear in this paper.