The implementation of safety codes and standards for hydrogen-based energy systems will require a scientific basis for evaluating credible safety issues and for developing safety codes and standards applicable to the future hydrogen infrastructure. The characterization of unintended releases from hydrogen components has been identified as an area of particular interest to codes and standards development. In the present study, measurements were performed in small-scale hydrogen leaks to characterize the dimensional properties and flow characteristics of the resulting ignitable hydrogen cloud. Specifically, the data will provide a technological basis for determining hazardous length scales associated with the formation of ignitable mixtures due to unintended releases. In contrast to previous studies where large-scale momentum-dominated releases were considered, the present study focuses on smaller-scale releases more characteristic of low-pressure electrolyzers, leaky fittings or O-rings seals where moderate pressure drops and lower flow rates might occur. A turbulent jet flow is selected as representative of releases in which the leak geometry is circular. Laser-based Rayleigh scattering is used to characterize the hydrogen concentration field downstream of the leak. Time-averaged mean and fluctuating hydrogen concentration statistics and the corresponding probability density distributions are presented and the results for a pure hydrogen jet are compared with literature data for other gases such as methane and propane. At the moderate Froude number (Fr = 268) flow condition studied, the global jet features such as H-2 centerline decay, jet spreading rate, and the mean and fluctuating H-2 concentration distributions are similar to other variable density jets in the momentum-dominated regime and follow similar scaling laws. The database presented should provide a good test for the validation of CFD models being developed to predict unintended hydrogen release scenarios. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.