Purpose <p>The use of water electrolysers (WEs) and fuel cells (FCs) is expanding in various sectors with significant potential for technological advancements. To ensure the environmental benefits of hydrogen systems, comprehensive life cycle assessments (LCAs) are essential, guiding the energy transition supported by policy and business. However, data on their environmental impacts are affected by gaps and uncertainties. This paper analyses the state of play of life cycle inventory (LCI) data for the hydrogen systems and formulates recommendations for improving the situation with regard to completeness and availability of data.</p> Methods <p>We analysed a sample of studies representative of the existing literature about LCA of hydrogen technologies, with a focus on the LCIs of FCs and WEs. We investigated several well-known databases available for LCA practitioners, researching key products and processes for the hydrogen value chain through the Global LCA Data Access network (GLAD), a repository of data sets. This analysis allowed us to identify the main areas where data gaps and uncertainties occur.</p> Results <p>Our analysis identified components, materials, processes, and life-cycle stages that are mostly affected by unavailability of high-quality data. This regards, for instance, the assembling of FC and WE stacks and their end of life (EoL) treatments. Data gaps affect specific treatments, such as the spray coating of cell electrodes, production of materials relevant for these technologies, such as graphite, specific polymers (e.g. Nafion), and extraction and refining of minerals and metals (e.g. Iridium). Furthermore, existing shortcomings in the datasets available in GLAD were identified, including possible risks of dis-harmonisation among datasets coming from different data providers. Our three-stage analysis reveals the difficulties and bottlenecks in developing LCA data for hydrogen technologies, proposing technical solutions, including the Life Cycle Data Network (LCDN) infrastructure, to improve data availability and promote life cycle thinking.</p> Conclusions <p>The scarcity of robust data on bill of materials and on input/outputs of production processes relevant for the hydrogen value chain currently hinders accurate results and interpretation of LCA studies in the sector. Our study highlights the importance of consistently formatted, well-documented, and quality-assured LCI datasets. If such LCI datasets will be available in the future, it will significantly positively impact LCA practices in the sector, enhancing much more complete, comparable, and transparent results. Our study advocates for industry and research contributions to the LCDN, promoting high-quality LCA results and interoperability between software, which could benefit policy and emerging markets beyond hydrogen technologies.</p>