Measurements were made in the near field of piston driven axisymmetric coaxial synthetic jets emanating from an orifice and a surrounding annulus of equal exit areas and cavity volumes. Piston velocity, amplitude, radial spacing between the orifice and the annulus, and exit angles had a strong influence on the dominant features of the flow. Flow visualization revealed three distinct topologies of the jet consisting of expanding, contracting and recirculating regions and doubling of the number of foci inside of the cavity compared to jet from the orifice alone. The direction of the swirl/rotation imposed on the mean flow was also dependent on the direction of the dominant foci. Interaction between flow from the orifice and the annulus amplified the azimuthal instability of ring vortices due to the periodic axial stretching and compression of the stream-wise vortex filaments. Bifurcation of ring vortices into elliptical lobes reported earlier for single cavity jet was also observed in the coaxial jet. The number of cellular structures however was considerably larger than the single jet case. Large excursions of the jets from the plane of symmetry were observed. Power spectra exhibited sub-harmonic distribution of energy due to coalescence of the vortices. Growth of jet width and decay of centerline velocity were strongly influenced by the spacing and frequency. Nomenclature A Amplitude of oscillations mm b Jet width (based on U^/2) D Diameter of the orifice mm d Radial spacing between orifice and annulus mm f Driver frequency fr Reduced frequency fA/Vp U0 Jet velocity at the nozzle exit mm/s U^ Maximum centerline velocity of the jet mm/s Vp Piston velocity mm/s x Streamwise distance measured from exit y Cross-stream/radial distance measured from the center of the orifice/nozzle 1 Associate Professor, Associate Fellow AIAA 2 Graduate Research Assistant, Student member AIAA Copyright 2002 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved