We report scalar field measuremets (viz. density, pressure and temperature) in axially symmetric under-expanded micro jets using the rainbow schlieren deflectometry (RSD) technique. The micro-RSD system used in this study provides non-intrusive measurements across the whole field at spatial resolution down to 25 μm. Experiments were conducted with air jets issuing from an orifice injector of 500 μm diameter at different supply pressures ranging from 60 psig to 110 psig. The transverse angular deflections of light rays determined from the schlieren images were used to obtain the local density field. Isentropic relationships were used to obtain pressure and temperature from the density field. Detailed analysis was developed to determine uncertainties in measurements of density, pressure, and temperature. Results show that the micro-RSD technique provides high quality data in under-expanded micro-jets, and that the micro-jets behave in a manner similar to the macro-jets. Copyright © 2008 by Copyright@ 2008 by Ajay K. Agrawal.