Thin films of molybdenum trioxide find wide applications in electronic display systems [1] and solid state microbatteries [2, 3]. The significance of the material arises rnainly due to its photochromism [4], electrochromic properties [5] and unique layered structure [6]. Various methods, such as vacuum evaporation [7, 8], reactive sputtering [9], d.c. magnetron sputtering [10] and rf sputtering [5], have been reported for the deposition of MoO 3 films. In the investigation reported here an attempt was made to prepare MoO 3 films by electron beam evaporation on well cleaned glass substrates, and to characterize them by smdying their structure, electrical and optical properties. The effect of heat treatment in air on the film properties was also studied. Thin films of MoO 3 were prepared by electron beam evaporation using a Balzers BA 510 E vacuum coating unit onto chemically and ultrasonically cleaned Corning 7059 glass substrates maintained in the temperature range 303-573 K, in a vacuum better than 5 x 10 .7 mbar (1 bar = 1 x 10 » Pa). The deposition rate, controlled by a quartz crystal thickness monitor, was 4 nms « . The thickness of the films under investigation was about 500 nm. The structure of the films was analysed by Philips X-ray diffractometer (XRD) with a scanning angle (20) range of 10-60 °. Copper K« ()~ = 0.1542 nm) target was used as the source of X-rays. The infrared (IR) transmittance was recorded using a Pye Unicam SP3-300 IR spectrophotometer in the range 200-1200cm -1. The optical transmittance was recorded using a Hitachi U-3400 ultraviolet-visiblenear infrared (UV-VIS-NIR) double-beam spectrophotometer in the wavelength region 300-1600 nm. The electrical conductivity was measured using the standard four-probe technique. The XRD spectra of the films deposited at different substrate temperatures are shown in Fig. 1. The films formed at 303 K exhibited amorphous nature. On increasing the substrate temperature (> 423 K), the films exhibited polycrystalline nature. The films showed (O k 0) and (0 k l) orientations indicative of MoO3 in c-/~ mixed phase. The films deposited at 523 K showed predominantly (0 k0) orientation. On thermal treatment in air (calcination) at about 653 K for 4 h the films exhibited only (0 k 0) orientation, representing the orthorhombic c-phase of MoO 3. The evaluation lattice parameters from the XRD profiles were a = 0.396 nm, b = 1.365 nm and c = 0.357 nm [11]. On transformation from c-/3 mixed phase to c-phase MoO3 displays a very strong (0 k 0) preferred orien¢'4