A pressure-sensitive adhesive (PSA) was prepared by mixing low- and high-molecular-weight polyisobutylenes (PIB). Peeling of the adhesive from polycarbonate was observed from the side and from below at three peel rates. A correlation between the peel force and a characteristic dimension of the peel front was obtained. According to the observations, the deformation of adhesive across the peel front is dominated by uniaxial extension. From model stretch experiments simulating the observed stretch history of adhesive, theoretical peel force values were obtained which agree well with the measured peel force values.