  We report the observation of many-body interaction effects for a homonuclear
bosonic mixture in a three-dimensional optical lattice with variable state
dependence along one axis. Near the superfluid-to-Mott insulator transition for
one component, we find that the presence of a second component can reduce the
apparent superfluid coherence, most significantly when it either experiences a
strongly localizing lattice potential or none at all. We examine this effect by
varying the relative populations and lattice depths, and discuss the observed
behavior in view of recent proposals for scattering from impurities and of
atom-phonon coupling for atoms immersed in a superfluid.
