The treatment of landfill leachate with physico-chemical and Fenton and Fenton-like processes was investigated. Additionally, the acute toxicities of the raw, the physico-chemical pre-treated, and the Fenton and Fenton-like oxidation treated leachates on activated sludge microorganisms are given (determined by the ISO 8192 (respiration inhibition test)). Pre-treatment with lime positively affected the biodegradability of the sludge by preventing inhibition. The Fenton and Fenton-like processes resulted in high chemical oxidation demand (COD) (>80%) removal performance from leachate samples. The inhibitory effects of the Fenton and Fenton-like reagents on activated sludge mixtures decreased. In addition to toxicity, the COD removal efficiencies and the operation costs of the applied processes were determined.