Does anyone know how XP reduction works? Will I get less XP if I use a higher HD animal as companion, than a lower HD one? (And if so, does this apply to summoned monsters as well? Summon Monster I/II/III/IV/V/VI .. etc)

Yes. Having companions/summons will often reduce the XP you get because your "average party level" will be higher than usual, which tells the game that your enemies will be that much easier to defeat. Higher level companions/summons will increase the XP reduction, and lower level companions/summons will decrease the reduction, sometimes even to the point of increasing XP earned (ie. if your high level character summons a level 1 familiar).

Ah so that is the difference that I noticed in single player. What about multiplayer I seem to gain level ups faster than the person that I am playing the game with. As a wizard I have a familiar and he as a barbarian doesn't, but the difference is significant. Perhaps another Bug?

in single player, having a companion (either henchman, familiar, animal companion or a summoned creature) that is of higher level results in lower XP, because XP earned by killing enemies is based on average level of your party. this also means that having companions of lower level than your own will result in more XP earned!
for example, i saved my game before leveling my fighter to level 15. i then killed a certain creature and received 13 XP. i reloaded and leveled up again, this time taking a level of wizard and summoned a level 1 familiar. killing the same creature now gave me 248 XP! that's quite a difference [img]smile.gif[/img] .

however, i don't know how XP are divided in multiplayer. i haven't played it much and even then i didn't really pay attention to it...

<font color="lawngreen" face="trebuchet MS"> :eek: I thought that animal companions/henchman etc. had no affect on XP at all... Im gonna have to test this out.</font>