Temperature profiles have been measured inside a counter-current mixer for the continuous hydrothermal synthesis of inorganic nanoparticles, at conditions (10-25 ml min(-1) superheated water, referred to a density of I g ml(-1), at 350-450 degrees C and 24.1 MPa, mixed with precursors at 10-20 ml min(-1)) used in work published by some of the authors and others. The superheated water cooled significantly before meeting the precursors, owing to internal transfer of heat through the wall of the inner tube to the products flowing around it. Consequently, the region immediately after the fluids had fully mixed was at a lower temperature than that determined from an overall heat balance. The flow of superheated water emerging from the inner pipe was characterised using the relevant dimensionless groups (Reynolds, Froude). (C) 2011 Elsevier B.V. All rights reserved.