This paper formulates power allocation policies that maximize the region of mutual informations/nachievable in multiuser downlink OFDM channels. Arbitrary partitioning of/nthe available tones among users and arbitrary modulation formats, possibly different for/nevery user, are considered. Two distinct policies are derived, respectively for slow fading/nchannels tracked instantaneously by the transmitter and for fast fading channels known/nonly statistically thereby. With instantaneous channel tracking, the solution adopts the/nform of a multiuser mercury/waterfilling procedure that generalizes the single-user mercury//nwaterfilling introduced in [1, 2]. With only statistical channel information, in contrast,/nthe mercury/waterfilling interpretation is lost. For both policies, a number of limiting/nregimes are explored and illustrative examples are provided.