The effect of nonrubber constituents such as proteins and lipids on tack of natural rubber (NR) was studied. Tack or adhesion at short contact time was determined for contact between crosslinked rubbers and glass plates. Various types of natural rubber, with or without nonrubber constituents, were prepared. Synthetic polyisoprene was used as a reference because of its chemical similarity. In this work, the impact pendulum test was carried out to study very short contact times t(c) of the order of milliseconds. The results show an increase of tack energy with t(c) for all the rubbers. This is attributed to an increase in true contact area with time and to a modification of the interfacial energy. Furthermore, the natural rubber without lipids and also polyisoprene exhibited the highest tack values while whole natural rubber remained always the less tacky. These results cannot be explained by differences in either the bulk viscoelastic properties of the crosslinked materials or their overall surface energy. The presence of a thin layer of varying composition at the surface of the rubbers appears to be the essential factor that affects the tack properties of the crosslinked materials.