role in the stepwise implementation of the disposal. Such monitoring however implies particular conditions for the sensing equipment to be deployed. The experience gathered with experimental setups in underground research laboratories (URLs) is very valuable to improve the reliability of sensors envisaged for this monitoring. In Belgium, many experimental setups have been implemented in the HADES URL. With its construction started in 1980, it is the longest operational facility in a clay formation in Europe that is dedicated to investigate the feasibility and safety of geological disposal of radioactive waste. Technical experts use it to develop and test technologies for constructing, operating and closing a waste repository in a deep clay formation. Scientists conduct large-scale experiments to assess the safety of disposal in poorly indurated clay. As a result, thousands of sensors have been installed in the HADES URL over the past four decades. Many of them are regularly providing measurement data up to now. To obtain an insight in the factors that determine their performance, a research programme has been launched to assess the sensor performance of the HADES monitoring setups one by one. This paper describes the methodology for this assessment. It is based on a systematic screening of each individual sensor according to a set of indicators. As a first case, the CLIPEX instrumented setup was selected. It was installed more than 20 years ago to monitor the clay host rock response to gallery construction works. The assessment provided the performance data of the different sensor types, which allowed to make a performance appraisal of the respective sensor types. Based on the results of this first case, the assessment methodology will be applied to the other setups to extend the performance database of the different sensor types.