This document provides experimental testing protocols for assessing critical factors driving perfor-mance degradation of water electrolysis cells and stacks that will be used in the framework of the funded project DELYCIOUS (Project number: 101192075). It corresponds to the deliverable D2.1 of WP 2 Lab scale testing.The protocol will be applied to three different electrolysis technologies: Proton Exchange Membrane Electrolysis (PEMEL), Alkaline Electrolysis (AEL), Solid Oxide Electrolysis (SOEL). Albeit the net chemical reaction remains the same in the three encompassed technologies, the nature of the materials and conditions employed differs. This translates in differences in the experimental conditions and protocols required to assess the performance degradation mechanisms affecting the three technologies. Never-theless, based on the similarity of the type of key degradation effects under examination, their out-comes and their means of determination, this document is defined as “quasi-harmonized testing pro-tocol”.These protocols are intended for use in the framework of the project to highlight key degradation mechanisms and their operational causes using advanced monitoring and diagnostic tools developed in the framework of DELYCIOUS, ultimately allowing to develop, within the duration of the project, operational scenarios to improve cell/stack lifetime.Their use in other contexts by both the research community and industry, for research and develop-ment (R&D) purposes as benchmarking aid is welcomed.A review of the three technologies, in terms of a detailed description of their functionality, materials employed at stack and cell level as well as the current developments, is outside the scope of this docu-ment and only definitions deemed necessary for understanding the protocol will be recalled in text.