The temperature and composition of the upper atmosphere of Titan have been inferred by observing an occultation of the sun by Titan, using the Voyager 1 ultraviolet spectrometer. The temperature is 176¿20 K near the evening terminator and 196¿20 K near the morning terminator. The major constituent is N2 with a density of 2.7¿0.2¿108 cm-3 at 3840 km. The mixing ratio of CH4 is 8¿3% at a radial distance of 3700 km near the evening terminator where [CH4>?1.2¿108 cm-3. On the morning terminator the [CH4>?1.2¿108 cm-3 level is about 20 km lower in the atmosphere. The acetylene mixing ratio above 3400 km is measured at the 1 to 2% level. Below 3300 km it decreases to between 0.1 and 0.3%. A layer of absorbing molecules, possible polymers, is present at both morning and evening terminators. Near the evening terminator it is located about 100 km lower in the atmosphere. A simple photochemical model suggests that the homopause is located at 3500¿70 km with an eddy diffusion coefficient of 1+2-0.7¿108 cm2 s-1, which decreases to ~103 cm2 s-1 in the lower stratosphere as [N2>-2/3.