A comparison was made between the codeposition behavior of NiMo, CoMo, and FeMo alloys on rotating cylinder electrodes when the molybdate concentration in the electrolyte was much lower than that of the iron-group species. More molybdate was codeposited with Co than with either Fe or Ni from an ammonia-citrate electrolyte at pH 7.4. During the NiMo codeposition, the molybdate species is mass transport controlled. Substitution of the nickel by cobalt in the plating bath does not influence the molybdenum deposition rate. However. a higher concentration of molybdate is found in the deposits because the rate of cobalt deposition is lower than that of nickel. On the other hand, substitution of the nickel by iron results in a dramatic lowering of the molybdenum deposition rate. These observations were described by a mathematical model which assumes that iron-group species can adsorb on the electrode surface, competing viith the molybdenum intermediate for free surface sites. Thus tile diminished partial current density of the molybdenum in iron containing electrolytes can be explained by a blocking mechanism due to the adsorbed iron intermediate.