10 Speed Derailleur on 9 Speed Groupset

[retrojon]

Any problem running a 10 speed rear derailleur on an Ultegra 6500 9 speed setup?

Jockey wheels seem to be different.

 

[legrandefromage]

I believe that you may have width issues if using anything other than a ten spd chain.

I am also led to believe that a 10 spd chain works ok with 9 spd...

 

[dbmtb]

Have no problems running a 7800 Dura-Ace rear mech on my cross bike with 9 speed STIs, cassette and chain.

10 speed rear mechs are cack in 8-speed systems though.