The formation of particle-stabilised emulsions by adding partially hydrophobised silica particles to surfactant-free oil-in-water emulsions (average drop diameter similar to 700 nm) stabilised by hydroxide ions adsorbed at the oil-water interface has been investigated. Nanoparticles (average particle diameter 18 nm) adsorbed onto the drops under alkaline conditions to produce particle-stabilised emulsions with the same drop size distribution as the surfactant-free emulsions. Unlike the surfactant-free emulsions, the particle-stabilised emulsions were stable even in acidic conditions. Strongly flocculated nanoparticles (average particle diameter 150 nm) adsorbed onto the drop surfaces under acidic conditions where the emulsions were destabilised, forming coarser particle-stabilised emulsions with micron-sized drops. (c) 2006 Elsevier Inc. All rights reserved.