the SRD 3.5 says:
Incorporeal Subtype: An incorporeal creature has no physical body. It can be harmed only by other incorporeal creatures, magic weapons or creatures that strike as magic weapons, and spells, spell-like abilities, or supernatural abilities. It is immune to all nonmagical attack forms. Even when hit by spells or magic weapons, it has a 50% chance to ignore any damage from a corporeal source (except for positive energy, negative energy, force effects such as magic missile, or attacks made with ghost touch weapons). Although it is not a magical attack, holy water can affect incorporeal undead, but a hit with holy water has a 50% chance of not affecting an incorporeal creature.
