An investigation of the iron-molybdenum binary system was undertaken to determine the effects of molybdenum on pitting and passive film behavior. Passive films, which were formed in a borate buffer solution, were depleted in molybdenum and were in general less protective than that on Fe. Alloying up to 15% Mo increased the resistance to pitting in a borate buffer solution plus 0. 1N KCl solution, but with less effect than observed for stainless steels. Also, the pitting resistance was observed to decrease with increasing temperature for each binary.