H-1 nuclear magnetic resonance (n.m.r.) relaxation data, determined via either wide-line H-1 n.m.r, or high-resolution solid-state C-13 n.m.r., were used to analyse the existence and extent of interphasic regions in (methylmethacrylate)-b-(n-butylacrylate) block copolymers. Results thus obtained have shown that the as-obtained copolymers are not at thermodynamic equilibrium. Some n-butylacrylate units are trapped in the hard microphase, whereas some methylmethacrylate units are trapped in the soft microphase. On annealing the samples at temperatures far above the glass transition temperature of the hard microphase, both blocks are allowed to diffuse and phase separate, so that thermodynamic equilibrium can be reached. Differences in the diffusion efficiency have been observed as a function of the hard block length and number of blocks.