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Abstract Three types of aging were found to exist. The factors determining the types were the temperature and the air-supply conditions irrespective of where the rubber was aged, in the tires or in the laboratory. This means that the aging characteristics of a tire part in the field can be properly predicted if these factors in a tire are taken into account in the laboratory evaluation. In the first type, Type I, the aging yields increased M 100 and reduced λ b closely following the reference relation, Equation (3), which holds for the rubbers crosslinked with increasing the amount of curatives. This type of aging was found at temperatures below about 80°C, under either oxidative or anaerobic conditions. The extents of the changes in λ b and M 100 were large under the former conditions and small under the latter. As for the aging in the heavy-duty tires, the sidewall and the belt showed this type, with the changes to considerable extents suggesting an oxidative aging. The oxidative condition for the sidewall is apparent. Although air supply to the belt seems difficult because of the interior position and the massiveness of the tires, the cords of air wicking type occupy a substantial section of the part shortening the permeation path of the inflating air to create an oxidative condition. This type of aging is governed substantially by crosslinking, so that aging yields the same effect on the λ b vs. M 100 relation as the one obtained in crosslinking with increasing curatives. In the second type, Type II, M 100 was either changed little or even reduced, while λ b was reduced. This type was observed in an anaerobic aging at temperatures higher than about 90°C. The extents of the changes in this type appeared relatively small. This type of aging in the heavy-duty tires was observed mainly in the belt-edge filler and sporadically in the belt. The belt-edge fillers are placed at the edges above the second and below the third belt layers with the thickness considerably larger than that of the belt rubber between the cords. This makes the air supply condition in the part virtually anaerobic. Furthermore, being in the midsection of the thick crown region and under a severe flexural condition, the part should operate at considerably high temperatures. The sporadic appearance of this type in the belt may be due to the thickness variation of the rubber layers including the underlying parts, together with the severe service conditions causing extensive temperature rise in the particular service. The characteristics of this type are speculated to come from extensive main-chain modifications like cyclic sulfide formations. In the third type, Type III, M 100 was increased, and λ b was reduced but to an extent larger than expected from the M 100 value on the basis of the reference relation. This type was obtained in the oxidative aging at temperatures above about 90°C. This type of aging was not found in the tire parts of the present study. The causes of this type are considered to be from appreciable amounts of chain scission in addition to crosslinking, due to oxidation at high temperatures.