Evaluating the performance of a wastewater treatment plant (WWTP) requires a good characterization of the biodegradable substrate entering the plant. As is generally acknowledged, the wastewater characteristics under rain or storm weather conditions vary significantly from dry weather conditions but this is rarely accounted for in modelling exercises. In order to address this defect, a measurement campaign was set up and samples were analysed using respirometric assays. In this paper, some hurdles regarding measurement campaigns under dilute wastewater conditions and a better exploration of the impact of using respirometric assays are described and discussed. The dependence of the heterotrophic yield on different substrates severely hampers the evaluation of the respirograms under dilute wastewater conditions. In addition, the low load conditions limit the application of the assays due to insufficient sensitivity of the experiment and uncontrolled oxygen inputs. The results clearly demonstrate the need for further research in order to allow for a proper evaluation of WWTP performance under rain and storm weather conditions.