Previously studies of hydrogen releases have focused on hydrogen jets through round nozzles, but leaks are more likely to result in jets through slots. In this study, the concentration decays along the jet centerline were measured for underexpanded jets from rectangular slot nozzles with aspect ratios (AR) of 1-12.6 for storage pressures up to 3.7 MPa. The mass fractions along the jet centerlines decay inversely with distance from the nozzle as with axisymmetric jets but with a faster decay rate. All the data collapses onto a single curve when normalized by the proper factor of the downstream location, except for the jets from square nozzles (AR1). The decay rates for jets from rectangular nozzles with AR greater than unity are significantly larger than for the jets from square nozzles. The jets from square nozzles behave somewhat like the circular jets from circular nozzles due to the symmetric geometry, so the decay rate is closer to that of circular jets relative to the jets from the other rectangular nozzles. This database will be useful for model validations for the modeling of jets from rectangular nozzles. (C)2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.