Reply #156: Ammonia is the favorite alternative. Very abundant on Jovians and near-Jovians.

[eppur_se_muova]

A mixture of ammonia and water might be a possibility, as in Hal Clement's novel "Fossil".

Clement also wrote a novel ("Close to Critical") in which the atmosphere was a mixture of water and sulfuric acid near the critical point, and one in which the atmosphere was gaseous sulfur (it wasn't Io). :D

http://en.wikipedia.org/wiki/Hal_Clement