You and target player exchange control of the creature you each control with the highest converted mana cost. Then exchange control of artifacts the same way. If two or more permanents a player controls are tied for highest cost, their controller chooses one of them.
9/16/2007 First creatures are exchanged, then artifacts are exchanged. It’s possible that the same artifact creature may be involved in both exchanges.
9/16/2007 No permanents are targeted by this effect, so permanents with Shroud may be exchanged this way.
9/16/2007 If one of the players doesn’t control a creature at the time the exchange would be made, that part of the effect does nothing, but the exchange of artifacts will still happen. Similarly, if one of the players doesn’t control an artifact at the time the exchange would be made, that part of the effect does nothing.