I have 13 THACO with a +2 sword and 11 with my bow. That means if an enemy has AC 0 I'll hit it 1/3 times with the sword and 1/2 times with the bow (roughly). Well, I go up against kobolds and xvarts and skeletons, whose AC I am SURE isn't as low as zero, and I REGULARLY go 6 or 7 swings without hitting them. Is anyone else going through this? It seems like bows are waaaay more accurate than close ranged weaponry, both in the party and with enemies. Does anyone have an explanation of this phenomenon?

You may be counting the 'cosmetic swings', which are just extra animations to make things look more exciting. I think there's an option to have the game report all your rolls, which should make it clearer what's going on.

This. Look under Options, Gameplay, Feedback. This answer also explains why bows seem so much more accurate - they only make the attack animation when they actually make an attack roll.

Thanks, that makes sense!

For BG2 / Tutu / Trilogy, you can use TobEx mod to remove the cosmetic attack animations if they really bother you. I think it looks better without them myself.

(Note however TobEx is recommended to be first in the mod install order, before the Fixpack(s), so you might have to uninstall and reinstall mods. Also pick carefully which TobEx options you select, as some are very close to cheats / ruleset changes. After installation you can change options by editing the INI files to your liking without reinstalling).

Actually I found the game to be less appealing without cosmetic attack animations.