Critical to advancing the uptake of olefin metathesis in leading contexts, including pharmaceutical manufacturing, is identification of highly active catalysts that resist decomposition. Amines constitute an aggressive challenge to ruthenium metathesis catalysts. Examined here is the impact of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), morpholine, -butylamine, and triethylamine on Ru metathesis catalysts that represent the current state of the art, including cyclic alkyl amino carbene (CAAC) and N-heterocyclic carbene (NHC) complexes. Accordingly, the amine-tolerance of the nitro-Grela catalyst RuCl(HIMes)(=CHAr) (; Ar = CH-2-O Pr-5-NO) is compared with that of its CAAC analogues and , and the Hoveyda-class catalyst RuCl()(=CHAr') (Ar' = CH-2-O Pr). In , the carbene carbon is flanked by an -2,6-EtCH group and a CMePh quaternary carbon; in , by an -2- Pr-6-MeCH group and a CMe quaternary carbon. The impact of 1 equiv amine per Ru on turnover numbers (TONs) in ring-closing metathesis of diethyl diallylmalonate was assessed at 9 ppm Ru, at RT and 70 °C. The deleterious impact of amines followed the trend NEt ∼ NH Bu ≪ DBU ∼ morpholine. Morpholine is shown to decompose by nucleophilic abstraction of the methylidene ligand; DBU, by proton abstraction from the metallacyclobutane. Decomposition was minimized at 70 °C, at which enabled TONs of ca. 60 000 even in the presence of morpholine or DBU, vs ca. 80 000 in the absence of base. Unexpectedly, HIMes catalyst delivered 70-90% of the performance of at high temperatures, and underwent decomposition by Brønsted base at a similar rate. Density functional theory (DFT) analysis shows that this similarity is due to comparable net electron donation by the HIMes and ligands. Catalysts bearing the smaller ligand were comparatively insensitive to amines, owing to rapid, preferential bimolecular decomposition.