The flocculation behaviour of poly(N-isopropylacrylamide) (PNIPAM) microgel particles, containing surface sulphate groups, has been studied as a function of the ionic strength, between 0.1 and 800 mM NaCl, and over the temperature range 25-60 degreesC. The critical flocculation temperature (CFT) of the particles was determined is a function of NaCl concentration. Three regions of NaCl concentration were established. Firstly, at very low values of [NaCl] (less than or similar to 25 mM), no CFT value could be determined; this implies that the interparticle electrostatic repulsion is sufficient to prevent any flocculation occurring. This remains the case, even at temperatures well in excess of the lower critical solution temperature for PNIPAM in solution, where the particles are essentially deswollen. Secondly, at intermediate [NaCl] (similar to25-100 mM), the CFT decreased strongly with increasing [NaCl]. In this region the electrostatic forces are weakened sufficiently, for the van der Waals forces to cause flocculation. Thirdly, at higher [NaCl] (greater than or similar to 100 mM), the electrostatic repulsion is screened out, and the CFT decreases linearly with temperature. The reason for this decrease is the fact that aqueous solutions of NaCl become increasingly poorer solvent environments for PNIPAM, with increasing [NaCI]. These trends are apparent also in the values determined for the hydrodynamic size of the stable PNIPAM particles as a function of [NaCl] and temperature. It is shown that the flocculation of the PNIPAM particles is consistent with a weak, reversible flocculation model. This is apparent, for example, from the fractal dimensions of the flocs (similar to2.0), determined from the power law used to fit the time-evolution of the hydrodynamic size of the floes, and also from the estimated depth of the minimum in the interparticle pair potentiation, based on the critical size of the primary particles where flocculation just begins to occur. (C) 2003 Elsevier B.V. All rights reserved.