TMAH (tetramethyl ammonium hydroxide) originating from etching and photo-developing processes was treated with Fenton oxidation followed by an activated sludge. Additionally, a Microtox test was performed to address any potential toxicity of TMAH against mixed cultures of microorganisms in the activated sludge. The Microtox test revealed that toxicity of TMAH against Photobacterium phosphoreum was highly effective showing 5% of EC50, but its toxicity was completely dissipated showing 100% of EC50 being recovered after being treated with Fenton reagents. BOD5 test showed that acclimated cultures to TMAH could readily decompose TMAH in an order of magnitude higher than that of not-acclimated culture. Feasibility tests showed that TMAH was readily biodegraded after being oxidized by the Fenton process, while TMAH fed directly into the activated sludge was laggardly decomposed during longer adaptation period. In the presence of acetic acid, activity of acclimated mixed cultures to TMAH was considerably reduced by dominant presence of predators competitively utilizing acetic acid.