H-1 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 dimethylsiloxane-styrene block copolymers. Results thus obtained have shown that copolymers having very short blocks possess relatively large interphases characterized by intermediate mobility. In addition, some styrene units are trapped in the dimethylsiloxane soft microphase. With increasing block molecular weights, the extent of the phase of intermediate mobility, as well as the percentage of styrene units trapped in the dimethylsiloxane soft microphase, decrease. In agreement with the conclusions previously deduced from d.s.c. experiments, and within the sensitivity of the n.m.r. technique, the samples with the longest blocks are sharply phase-separated.