Recent "tack" experiments on Newtonian liquids have shown that the force versus gap measurements obtained by pulling apart two surfaces separated by a thin liquid film result from a complex and dynamic balance between the viscous force resisting the separation and the spring force exerted by the compliant load cell of the instrument [Tirumkudulu et al. Phys. Fluids 15(6), 1588-1605 (2003)]. Here, we present similar experiments with waterborne adhesives that are colloidal dispersions of soft polymer spheres dispersed in water. By employing a simple power law to describe the complex rheology of the waterborne adhesives and a lubrication approximation for the viscous force, we predict pull-off forces in close agreement with those observed experimentally. However, experiments with high particle concentration adhesive and/or large separation rates cause cavitation in the gap, resulting in forces that are lower than predicted. Finally, a simple analysis for adhesives of two different rheologies shows that the tack test may result in contradictory conclusions, since the forces are sensitive to small variations in the instrument settings. (C) 2003 The Society of Rheology.