The adsorption of polymer on the surface of colloidal particles can by means of ‘steric stabilisation’ produce a great resistance to fluocculation, even at high electrolyte concentration in the dispersion medium. The use of a polyelectrolyte as a steric stabiliser is described. In this system large changes in polymer conformation and solution properties can be brought about by alteration of solution pH and indifferent electrolyte concentration. The modification of the stability of colloidal carbon black (Regal SRF) dispersions in KCl solution, in the concentration range 0.25–1.0 mol dm^–3 and from 283 to 303 K, arising from adsorption of poly(acrylic acid)(molecular weight 150000) has been investigated as a function of dispersion medium pH. The dispersion stability has been related to both the polyelectrolyte solution properties and the extent of coverage of the colloid by adsorbed polyelectrolyte.