There is apparently no definitive spelling rule to determine when to use -able or -ible at the end of a word. The primary heuristic is to evaluate the root word as follows:
- If the root is a full word, use -able.
- If the root is not a full word, use -ible.
- If the root is a word that ends in ‘e’, drop the ‘e’ and append -able.
- If the root is a word that ends in ‘y’, change the ‘y’ to ‘i’ and (thus) use -ible.
- If the root must be modified in any other way, use -ible.
There is a non-trivial number of exceptions. Also, it has been observed that most words require the -able ending, so if you have to guess, the odds favor -able.
http://owl.english.purdue.edu/handouts/grammar/g_spelible.html [broken link]