The preparation and physico-chemical characterisation of a novel thermosensitive microgel dispersion of anionic poly(N-ethylacrylamide), poly(NEAM), is reported. Dispersions were prepared by the single-step emulsion polymerisation of N-ethylacrylamide in water, in the presence of N,N＇-methylenebisacrylamide as a cross-linking agent and ammonium persulfate as the initiator. Transmission electron micrographs of the microgel dispersions showed that the reaction time greatly influenced the physical characteristics of the resultant microgel particles, with monodisperse spheres, having a mean diameter of 414 +/- 21 nm, being obtained following a 24-h preparation. Using high sensitivity differential scanning calorimetry (HSDSC) a kinetically limited phase transition was observed for a 2.6% (w/w) dispersion, having an excess specific heat capacity maximum at 78.2 degrees C (at a scan rate of 60 K h(-1)). The thermodynamic parameters and turbidimetric changes (as a function of temperature, pH and electrolyte concentration) associated with this transition are reported. In the presence of 1 mol dm(-3) sodium chloride, the temperature at which this conformational transition occurs decreases with a corresponding increase in the calorimetric enthalpy of the transition. A reversible aggregation of the microgel particles was also observed on heating a 0.7% (w/w) dispersion to 80 degrees C in the presence of 3 mol dm(-3) sodium chloride. Experimental measurements of the reduced viscosity, eta(red), of a 3% (w/w) dispersion, as a function of temperature and electrolyte concentration, provided further evidence of a temperature-induced conformational change with respect to the poly(NEAM) chains, resulting in a more compact conformation.