If it can be seen, the constituents of a surface are generally determined by detailed observations of spectra. These measurements indicate that sulfur is the primary constituent of surface of Io.

An idea of the interior composition can be guessed from the density of Io, which is about 3.5 g/cm3. The density of typical terrestrial iron is about 5, and that of silicate rock is about 3 g/cm3, so the inside of Io must be made out of rocky material and sulfur.

Deep inside Io is probably a core made of heavier metal elements such as iron. This is where the magnetosphere of Io may be produced.

The composition of Io is very different from the other icy satellites and suggests a certain path for Io's evolution.