High Arcana

[RisnDevil]

So now that every partyhas magic user that is a mystic theurge I have a question, the High Arcana abilities of the archmage, can he use them with divine spells?

 

[Anabstercorian]

Depends on the High Arcana ability:

Arcane Fire: No, it specifies arcane spells.
Arcane Reach: You sure can. Go ahead and Harm those poor bastards.
Mastery of Counterspelling: You can.
Mastery of Elements: It specifies arcane spells, so no.
Mastery of Shaping: You can. Makes for some rather impressive Flame Strikes, I think.
Spell Power: You can, but if you make it apply to your Cleric or Druid levels, it won't apply to your Wizard levels. Good way to make up for your caster level deficiency, though.
Spell-Like Ability: No, it specifies arcane spells and arcane spell slots.

 

[Alratan]

I'm not sure about Spell Power, I think it could well apply to all your caster levels.

Spell Power: This ability increases the archmage’s effective caster level by +1 (for purposes of determining level-dependent spell variables such as damage dice or range, and caster level checks only). This ability costs one 5th-level spell slot.

There is no particular restriction to applying to only one class - it says you get an increase to your caster level, just as an orange Ioun stone does.