A model of the Io plasma torus has been constructed using the in situ plasma measurement of Voyager 1. A sharp gradient in plasma temperature of ~7¿105 K RJ-1 at 5.7 RJ divides the torus into two parts, a cold inner region, where the ions are closely confined to the centrifugal equator, and a warm outer region, which includes the orbit of Io and has a thickness scale height of 1 RJ. The outer edge of the warm torus is defined by a drop in plasma density near 7.5 RJ. The bulk motion of the plasma, i.e., the average velocity vector, is within 1% of the value expected on the basis of strict corotation in the inner part of the torus but probably deviates by 5 to 10% from corotation outside the torus. This breakdown from corotation may occur at the outer boundary of the warm torus. The energy per charge spectra show well-resolved peaks in the inner part of the torus but strongly overlapping peaks;in the outer part. In the inner torus there is a significant variation in the abundances of different ionic species over spatial scales <104 km. However, in the plasma sheet of the middle magnetosphere the ionic composition appears to be uniform from 12 to 42 RJ and is strongly dominated by ions with a ratio of atomic mass to charge of 16. Theses ions are most probably some combination of O+ and S2+ ions. One consequence of the observation is that the Alfven speed is uniformly low in the outer part of the torus, with values less than 250 km s-1.