The paper discusses vertical profiles of salinity and longitudinal velocity observed in an 11-km study reach of the Upper Clyde Estuary in Scotland. Corresponding tide level measurements and calculated area mean velocities are also presented. The data indicate that the upper estuary was highly stratified during two survey periods and was almost well mixed during a third. This is broadly consistent with an earlier classification of the estuary as a type 2b within the scheme proposed by Hansen and Rattray (1966). When the upper estuary is stratified, the salinity data demonstrate the existence of a classical multi-layer flow structure consisting of an approximately well-mixed upper layer, an approximately well-mixed lower layer, and an interfacial layer containing a large vertical density gradient. The velocity data, obtained from one of the stratified flow periods, reveal a commensurate velocity field with water moving seawards in the upper layer and landwards in the lower layer. As a consequence of strong vertical shear, longitudinal dispersion coefficients of the order of 1000 m2/s are estimated. Since the data only describe conditions on a limited number of occasions, they are more illustrative of possible conditions than representative of typical ones. Nevertheless, they clearly indicate that water quality models of the upper estuary need to be capable of resolving the depth variation of environmental parameters.