Viscosity reduction of Natural Rubber (NR) is a common problem in the entire rubber industry, and especially in tires where viscosity stabilized grades of NR are too expensive and not even available in sufficient quantities. Mechanical mastication is rather inefficient, so chemical peptizers are widely used. Their effect in viscosity reduction is very good even in small dosages as 0.1-0.2 phr, but they also produce a lot of short chain material which results in poorer dynamic properties and increased sticking on the production machines. Moreover, due to changes in the mixing procedure over the last years, peptizers are not fully utilized any more. The strongest effect is achieved at temperatures above 120degreesC when oxidative chain scission catalyzed by the so called boosters becomes dominant. At this time often the fillers are added already, thus the booster is deactivated. To overcome this problem, Schill + Seilacher recently introduced a new concept of NR viscosity reduction [1]. The new material, Struktol HIT 105 avoids the drawbacks of chemical peptizers but is still zinc based. With view to the upcoming restrictions on zinc by the EU a zinc free version of the new peptizer was developed, Struktol XP 1440.