I have just reached Baldur's Gate and Imoen has just reached level nine(80,000 exp i think)and I have TOTSC installed. I have decided to dual class her to a mage and I was wondering if it actualy possible with the TOTSC exp cap. I also wanted to hear other peoples opinions on what type of mage to dual to. The only other mage in my Party is Dynaheir (level 9 Invoker)

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Well, as Kaltia pointed out you're in the wrong forum ;)
Anyhow, IMHO that's a bit late to dual class her! It will take ages for her to get her thieving abilities back and she'll be so weak compared to the rest of your party as she goes through the first few mage levels! I'm not sure if you can still manage to get her high up enough to get back your thieving skills, but even if you do, it's a waste of time I think. Most people dual-class her much earlier. Still, if you're decided on it, I wish you good luck! ;)

A sort of cheesey way is to start with your lvl 9 character anew...Imoen is a high level thief straight away then, so you could dual-class her. However, that IS the cheesy way, so I don't really like doing it.

And youd still have the cap to contend with. Level 9 thief is 110K XP. That leaves 51K XP or enough for mage level 6. Nearly 7 in fact, but you need 10, which is 250K XP. Which is impossible anyway, given that the ToTSC cap is only 161K XP.

You could dual at level 7 and be able to do it, but no higher than that.

If you really want to do it you could get a cap remover, but I dont have a link to a BG1 one.

And you cant dual to a specialist mage, so you have no choice but to go with a standard mage.

In BGI I always dual-class Immy at Level 5 thief. That way, I have a junior mage in the party that can use wands, etc.