The UVic Writer's Guide

• If the root exists independently of the compound (fix/fixable).
• If the root ends in "y" (deny/deniable).
• If the root's derivatives use "a" (separate/separable)
• If the root ends in a hard "c" or "g" (despicable, huggable).

Use -ible:

• If the root is not an independent word (visible)
• If there is a related -ion form (permission/permissible)
• If the root ends in -ns or -miss (reprehensible, dismissible)