Imidazolium Brønsted acidic catalysts substituted with ester/amides have been assessed for both antimicrobial toxicity and biodegradation. Low toxicity to a screen of 20 microbial strains (12 fungi and 8 bacteria) was demonstrated. Imidazolium salts incorporating either ester or amide groups at N-1, C-2, C-4 and C-5 did not pass the readily biodegradable test (ISO 14593). Catalyst selection based on Traffic Signal Light classification of performance, (eco)toxicity, and efficient synthesis is described.