Abstract: A stimulus-responsive formulation of stimulus-responsive polymer particles in aqueous composition has a rheology-stimulus profile (e.g. rheology-temperature) whereby it exhibits certain rheological behaviours over a range of temperatures, which is controllable by copolymerising a stimulus-responsive polymer-forming monomer, such as N-isopropylacrylamide, with a certain proportion of second monomer having weak acid functionality (such as acrylic acid) to form the stimulus-responsive microgel particles of the formulation and by addition of a base to the formulation to neutralise a portion but not all of the weak acid functionality. Thus a stimulus-responsive formulation may be controlled to exhibit gel-to-liquid-to-gel like behaviour with increasing temperature.