Monoterpenes are antimicrobial compounds widely distributed in vegetable biomass, whose inhibitory potential for anaerobic digestion is underestimated. In this research, the toxic effect of limonene and fenchone, two of the main monoterpenes present in vegetable biomasses, and those of 4-terpineol, α-terpineol, and p-cymene, compounds described as main metabolites of limonene degradation, have been assessed. Methane production was totally inhibited at dosed of 1000 mg L−1 of fenchone and limonene and at 600 mg L−1 of p-cymene and 4-terpineol. Based on the methane production rate, the inhibition followed the next trend: α-terpineol < < fenchone < limonene ≈ p-cymene < 4-terpineol. Regardless of dosed concentration, monoterpenes were mostly degraded at the end of the experiment (>85%), except p-cymene at 600 mg L−1. Therefore, monoterpenes could entail a high risk of inhibition that can be aggravated by the difficulty to accurately follow their concentration and by the scarce information on their effect on anaerobic process.