This study aimed at analysing the utilization of an electronic nose (e-nose) to serve as a specific monitoring tool for anaerobic digestion process, especially for detecting organic overload. An array of non specific metal oxide semiconductor gas sensors were used to detect process faults due to organic overload events in twelve 1.8-L anaerobic semi-continuous reactors. Three different load strategies were followed: (1) a cautious organic load (1.3 gVS L(-1) day(-1)); (2) an increasing load strategy (1.3-5.3 gVS L(-1) day(-1)) and (3) a cautious organic load with load pulses of up to 12 gVS L(-1) day(-1). A first monitoring campaign was conducted with three different substrates: sucrose, maize oil and a mix of sucrose/oil during 60 days. The second campaign was run with dry sugar beet pulp for 45 days. Hotelling's T(2) value and upper control limit to a reference set of digesters fed with a cautious OLR (1.3 gVS L(-1) day(-1)) was used as indirect state variable of the reactors. Overload situations were identified by the e-nose apparatus with Hotelling's T(2) values at least four times higher in magnitude than the upper control limit of 23.7. These results confirmed that the e-nose technology appeared promising for online detection of process imbalances in the domain of anaerobic digestion.