At bioreactor landfills, biodegradation parameters are carefully controlled to achieve faster stabilization of the waste mass. For example, liquid (leachate) injection into the waste mass is used to increase waste moisture content and thereby enhance waste degradation. The objective of this study is to validate the use of non-intrusive two-dimensional electrical resistivity imaging (ERI) for monitoring a liquid injection system, and to determine the relationship between measured resistivity and water content variations in waste. Temporal variations of waste electrical resistivity, during leachate recirculation, are mainly linked to water and ionic content variations. Two-dimensional ERI monitoring was carried out at a municipal solid waste landfill using a multi-channel resistivity meter to follow transient resistivity variations during leachate recirculation in the waste mass. Resistivity variations mainly correspond to variations in water content, and to temperature variations resulting from the injection of cold leachate into the warm waste mass. Laboratory experiments can be used to calibrate water content variations in the waste as a function of variations in electrical resistivity. Although such an approach is realistic for short measurement periods (hours), it is not reliable over longer time periods (months) since other parameters, such as biodegradation of the waste, may also influence its electrical resistivity.