We analyze the peeling properties of an uncross-linked pressure sensitive adhesive. 90 degrees peeling master curves on Pyrex(TM) and PMMA (polymethylmethacrylate) are constructed. The shift coefficients a, are compared with the ones obtained from rheometrical shear tests. With our machine, the peeling front is kept fixed, enabling us to observe the mechanisms of deformation of the adhesive. We count four different mechanisms of peeling in cohesive failure, and three in interfacial peeling (the last being unstable); they correspond to various slopes that we identify. The flow patterns at slow reduced velocities are two-dimensional. Then they undergo transitions to three-dimensional periodic complex hows, due to instabilities in the flow of thin adhesives. Interpretation of these peeling master curves are discussed in terms of rheology and physico-chemistry. It appears necessary to take into account the elongational properties of the adhesive, as well as the surface energy properties, to predict adhesion.