The Novor vulcanizing systems are based on adducts of a nitrosophenol with a bis- isocyanate. The Novor systems were developed to provide reversion-resistant cures, in particular in NR at high cure temperatures. For many years Novor vulcanizates have also been associated with fatigue resistance that increases upon heat ageing. Historically, they have been used together with accelerators, such as ZDMC and TMTM, but these are of increasing concern due to their association with nitrosamine generation. However, simple replacement of ZDMC or TMTM with accelerators considered safe as regards nitrosamines leads to a reduction of the improvement in aged fatigue life seen with a traditional Novor-based system. Using an experimental design approach, a new addition (Novor 800) to the Novor cure system family has been developed. When paired with a nitrosamine-safe accelerator, TBzTD, this new system provides significant improvements in aged fatigue lifetimes compared to previous combinations, coupled with a reduction of 20% in compression set. These results suggest that Novor 800 could be appropriate for applications for which the Novor systems were not previously considered.