The work sets out to determine a suitable analytical model to represent the decay of axial velocity from underexpanded sonic jets. Experiments are carried out using shadowgraphy and laser Doppler anemometry to determine the near field jet structure and the axial velocity distribution for the complete field down to 30 mls over a range of exit nozzle diameters, exit pressure ratios and jet gases. Comparisons are made with existing data and the best models available for fully expanded jets. It is shown that the decay law proposed by Kleinstein, with suitable choice of parameters, gives a good representation when the exit diameter is modified by the exit pressure ratio.