The diagnostics study on supersonic argon/nitrogen plasma jets expanded into a low-pressure test chamber is carried out by means of emission spectroscopy and enthalpy probe measurement techniques. The spatial distributions of electron density, temperatures, and associated shock structure effects in plasma jets are investigated in conjunction with their direct dependency upon the chamber pressure. The experimental results show the occurrence and the position of different zones; i.e., supersonic expansion, stationary shock waves and subsonic jet at pressures below 51 kPa. Flow fluctuations due to the oblique shock wave at 39 kPa background pressure are observed and discussed. The electron density profiles show variations along the plasma axis that coincide with the position of the shock waves. The experimental results show the transition from the moderately under-expanded to the strongly under-expanded jet structure induced by lowering of the chamber pressure.