Skim rubber (SR) was obtained by coagulation of the serum phase separated in the course of preparation of concentrated natural rubber (NR) latex. Saponified skim rubber (SSR) was prepared by soaking of SR with aqueous sodium hydroxide. The effect of non-rubber components on the thermal oxidative degradation of SSR was analyzed under mild and heating conditions for SSR and deproteinized natural rubber (DPNR) as a model of purified NR. The effect of gel, ash, ester, nitrogenous and heavy metal on aging properties was analyzed for SSR and DPNR comparing with SR and NR coagulated from high ammoniated latex (HANR), respectively. SSR contained Cu ion of 37 ppm even after purification of SR by saponification, although other transition metal ions were not detected. The accelerated decomposition of SSR was presumed to be due to the activation of residual Cu ions as well as the decrease of nitrogenous substances acting as antioxidants after saponification. The effect of Cu ions was confirmed by heat-aging test at 140 °C for 150 min for DPNR mixed with Cu compounds such as [Cu(NH3)4]2+, CuSO4 and CuO. Butylated hydroxytoluene (BHT) was not effective to prevent the thermal oxidation of SSR samples, although it was effective for SR. This suggested the influence of residual Cu ion activated by treatment of SR with aqueous sodium hydroxide.