Using scanning tunneling microscopy and low energy electron diffraction, we show that the structure and morphology of cobalt islands deposited on clean, sulfur- and oxygen-covered Mo(110) at 400 K strongly depends on the nature of the interface. Two-dimensional islands of cobalt grow on clean and S-covered Mo(110), whereas three-dimensional islands grow on O-covered Mo(110). The islands on the clean surface are hexagonal, reflecting the structure of the bulk Co lattice. A square lattice is formed on the S-covered surface that is the result of significant S-Co interactions. The difference between the growth mode of the cobalt on sulfur- and oxygen-covered Mo(110) and the difference in island shape on clean and S-covered Mo(110) are both explained in terms of microscopic bonding differences at the interface related to the degree of charge transfer to the O and S from the Mo surface.