The effects of surface treatment and bulk elastic properties on the debonding mechanisms of soft, thin adhesive layers have been investigated. The debonding mechanisms of two well-characterized pressure-sensitive adhesives were studied by performing probe tack experiments. These experiments consist of a bonding phase, where a flat steel punch is brought into contact with the adhesive layer, and a subsequent debonding phase, where the probe is pulled away from the surface. Effects due strictly to the surface were studied by modifying the probe surface with a grafted poly(dimethylsiloxane) monolayer. The surface properties are described by a critical energy release rate, G(0). The series of debonding events were governed by the quantity. G(0)/E, where E is the elastic modulus of the adhesive. As this quantity increased, the adhesive failure modes shifted from (A) interfacial crack propagation to (B) cavitation within the adhesive layer followed by rapid adhesive detachment of the walls between cavities to (C) cavitation followed by extension of the adhesive to large strains.