The electrochemical and optical properties of Lithium phosphorous oxynitride (Lipon) thin films have been studied with an emphasis on the stability window vs. lithium metal and the behavior of the Li/Lipon interface. Impedance measurements made between -26 and 140 degrees C show that Lipon exhibits a single, Li+-ion conducting phase with an average conductivity of 2.3 (+/-0.7) x 10(-6) S/cm at 25 degrees C and an average activation energy of E(a) = 0.55 +/- 0.02 eV. No detectable reaction or degradation was evident at the Li/Lipon interface, and linear sweep voltammetry measurements on three-electrode cells indicated that Lipon is stable from 0 to about 5.5 V with respect to a Li+/Li reference. The complex refractive index of Lipon was measured by spectroscopic ellipsometry. Optical bandgaps of 3.45 and 3.75 eV were obtained from the ellipsometry data and from optical absorption measurements, respectively.