Microwave discharges with powers around 120W at f = 2.45 GHz created in low pressure argon gas at 0.1–3 mbar were investigated. The microwave energy is fed into a coaxial structure with the plasma as the outer conductor in such a way that a spatially extended surface wave was created. This linear geometry is advantageous for large area plasma processes. The discharge itself was examined spatially resolved by several kinds of optical emission spectroscopic methods. The absolute line intensity of argon 4p levels was measured and the electron temperature could be determined from this with the help of a collisional radiative model. Additionally, the absolute continuum radiation intensity method was employed to determine the electron density.