Experimental studies were conducted on an annular mixing layer which evolved from freely expanding adiabatic circular jet produced by a conical nozzle. The mixing layer region shrouding the potential core was explored at four different jet-exit Mach numbers - 0.5, 0.6, 0.7 and 0.8. Pressure measurements were carried out by means of a miniature Pitot probe attached to an automated traverse. Mean velocity profiles were obtained from the radial distribution of pressures across the annular mixing layer thickness at various axial locations. The iso-velocity curves, plotted in longitudinal plane, exhibited that the velocity in the line of the nozzle lip was always about 60% of the nozzle exit velocity for the entire range of Mach numbers studied. The mixing layer thickness, assessed in terms of vorticity thickness, was found to grow linearly with the longitudinal distance downstream of the nozzle. The mixing layer growth rate was found to decline with increasing Mach number. However, the mixing layer was found to possess a strong self-similarity with a unique profile common to all the Mach numbers. The entrainment of mass into the mixing layer was found to increase along the streamwise distance at a constant rate depending on the nozzle-exit Mach number - increase in the Mach number resulting in decrease in the entrainment. The present work also suggests that nozzle with higher convergence angle tends to enhance the spread of the mixing layer along with the entrained mass.