From the Rules of the Game (if desired):
"Though you cannot cast a spell as an attack of opportunity, you can use some spells as attacks of opportunity. If you're holding the charge from a touch spell, you can try to touch a foe as an attack of opportunity. Some spells last long enough to let you to make several melee attacks. Spells such as chill touch and produce flame prove useful for attacks of opportunity if someone provokes an attack of opportunity from you."
"Though you cannot cast a spell as an attack of opportunity, you can use some spells as attacks of opportunity. If you're holding the charge from a touch spell, you can try to touch a foe as an attack of opportunity. Some spells last long enough to let you to make several melee attacks. Spells such as chill touch and produce flame prove useful for attacks of opportunity if someone provokes an attack of opportunity from you."