The addition of traces of certain divalent metal ions (Ca2+ and Mg2+) to aqueous solutions of sodium alginate increases their viscosity to a high extent. Further increase in the concentration of the added metal ions causes gelation and precipitation of the metal alginate salt. The effect of adding the divalent metal ions on the viscosity of sodium alginate solutions (before gelation or precipitation stages) was investigated via rheological and viscosity measurements. The optimum concentrations of the divalent metal salts were determined. Calculated traces of divalent metal salts were added to pastes of low-viscosity sodium alginate (5%) to increase its viscosity and subsequently decrease the amount of sodium alginate required for use in screen printing of cotton fabrics with reactive dyes (i.e., decreasing cost). For the sake of comparison, a comparativey was conducted by preparing other series of printing pastes thickened with low-viscosity sodium alginate (8%) and high-viscosity sodium alginate (2.5%). These printing pastes were applied to cotton fabrics by using the same technique. (C) 2001 John Wiley & Sons, Inc.