The viscosity and shear strength of pressure-sensitive adhesives based on natural rubber (standard Malaysian rubber grade L) were Studied with gum rosin and petroresin as the tackifying resins. Effects of the concentration of the tackifying resin and the molecular weight of rubber on the two properties were systematically investigated. Toluene was used as the solvent throughout the study to prepare the adhesives. The viscosity and shear strength of the adhesives were determined with a rotary viscometer and a texture analyzer, respectively. For the shear test, a hand coater was used to coat the adhesives on the release paper substrate to provide coating thicknesses of 60 and 120 pm. The results indicated that the viscosity increased with the resin loading and molecular weight of rubber increasing. The viscosity of the adhesive prepared from petroresin had a higher value than that of the gum-rosin-based adhesive. The shear strength of the adhesives decreased gradually with increasing resin content for both tackifying resins and coating thicknesses, and this observation was attributed to the decrease in the cohesive strength due to the dilution effect of the resins. However, the shear strength passed through a maximum at a molecular weight of rubber of 8.5 x 10(4) for both resins. The gum-rosin-based adhesive consistently showed higher shear strength than that of the petroresin/natural rubber adhesive because of the better cohesiveness and compatibility of the former system. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 4079-4083, 2008