In this work the anaerobic co-treatment of the leachates produced during urban solid biodegradable wastes composting diluted with municipal wastewater, was investigated. Leachates produced during the first 30 days of garbage composting contained 102.1 g COD L^-1, 20 g VFA L^-1, acid pH and 0.64 g NH₄⁺-N L^-1. Instead, leachates that remained for 3 months the composting plant after being produced contained 15.8 g COD L^-1, 2.35 g VFA L^-1, a pH near neutrality and 5.36 g NH₄⁺-N L^-1. An anaerobic zeolite packed filter reactor was used for the biological treatment and was operated at a constant HRT of 1.5 d. Leachates were diluted with municipal wastewater (0.4 g COD L^-1) to obtain feedings with organic matter concentrations ranging from 1.8 to 13.1 g COD L^-1 and organic loading rates from 1.2 to 8.7 g COD L^-1 d^-1. Removal efficiencies attained increased from 79.5 to 91.5% and methane productivity from 0.23 to 1.84 L L^-1 d^-1, rendering 0.48 L CH₄ per g consumed COD, equivalent to 118 kJ L^-1 of fed influent.