The aim of the presented work was to study the methane production limits and to determine optimal conditions for flexible operation of an anaerobic reactor in order to set up an operational strategy. Punctual overloads were conducted in a laboratory-scale anaerobic reactor with readily biodegradable solid substrates, and the influences of overload intensity, baseload value and substrate used were investigated. A maximal daily value around 1000 mL/L of reactor for methane production has been assessed. This value did not evolve significantly during experiment time, and conditioned the persistence of overloads as well as the flexibility margin on the reactor, which ranged from +25% to +140% on daily production. Results highlighted the fact that for a maximum flexibility, low organic loading rates are better to work with on this type of reactors.