Barium strontium titanate (Ba(1-x)SrxTiO3) nanopowders have been prepared at both bench- and pilot scales using a fully hydrothermal continuous-flow system. A mixed solution of barium nitrate (and/or strontium nitrate) with titanium bis (ammonium lactato) dihydroxide was mixed in-flow with a sodium hydroxide stream before meeting a pre-heated flow of supercritical water at a counter-current mixing point which resulted in the crystallisation of Ba(1-x)SrxTiO3. The obtained nanopowders were characterised by ICP-MS, powder X-ray diffraction, transmission electron microscopy and Raman spectroscopy. All products were found to be sub-stoichiometric with the total M2+:Ti ratio varying between 0.8:1 and 0.96:1, attributed to the formation of divalent metal carbonates preventing full incorporation of the M2+ ions into the perovskite structure in the extremely short reaction times employed. The obtained products were found to adopt the expected perovskite structure, while increasing strontium content led to a decrease in both particle size and lattice parameter. Products with target compositions of Ba(1-x)SrxTiO3 (0 <= x <= 1) were produced for the first time by a fully hydrothermal route at both the bench-scale of similar to 5 g h(-1) and pilot-scale at similar to 80 g h(-1). (C) 2015 Elsevier B.V. All rights reserved.