Numerous bioactive compounds in plant-derived products are secondary toxic metabolites produced by plants as a mechanism of defence against herbivores and invading microbes. Among them, essential oils (EO), which are known for their antimicrobial effects have been proposed as modulators of rumen fermentation. However, there is little information on their dose–response effects on the rumen ecosystem. Using five natural EO from Thymus vulgaris, Origanum vulgare, thymol chemo-type of O. vulgare, Cinnamomum verum, and Anethum graveolens, and three pure constituents thymol, carvacrol, and cinnamaldehyde, we examined the in vitro response of the rumen microbial ecosystem evaluated through production curves of volatile fatty acids (VFA), ammonia, and gas. Three types of EO molecules were discriminated, being carvone (a terpene), thymol, and carvacrol (phenolic EO originated from the terpene pathway), and cinnamaldehyde (a phenolic EO originating from the phenyl-propane pathway). The carvone-based EO produced a linear non-threshold profile that negatively affected end-products of fermentation. A threshold profile, characterized by a virtual stop of fermentation when doses were higher than the threshold level, occurred for thymol/carvacrol-