Three hydrolyzed-starch–polyacrylonitrile (HSPAN) copolymer materials tested as gel–water dispersions in a Brookfield rotating spindle viscometer exhibited velocity slip at solid boundaries and a yield stress. Dispersions with 0.5% HSPAN concentrations were about 20,000 times more viscous than water when the shear stress surpassed the yield stress and viscous power-law flow ensued. The apparent viscosity of an HSPAN gel–water dispersion was reduced by nearly an order of magnitude when tap water was substituted for deionized water. The apparent viscosity of the HSPAN gel–water dispersions decreased by about 30% after the fluid was continuously maintained at a temperature of 80°C for 1 week, and by an order of magnitude or more after 3 weeks under those conditions. In natural convection tests, the yield stress enabled an HSPAN gel–water dispersion to withstand greater temperature differences across a horizontal layer before inception of natural convection than a fluid without one. © 1993 John Wiley & Sons, Inc.