Pt black and PtRu black fuel cell anodes have been modified with Mo oxide and evaluated in direct methanol, formaldehyde and formic acid fuel cells. Mo oxide deposition by reductive electrodeposition from sodium molybdate or by spraying of the fuel cell anode with aqueous sodium molybdate resulted in similar performance gains in formaldehyde cells. At current densities below ca. 20 mA cm(-2), cell voltages were 350-450 mV higher when the Pt catalyst was modified with Mo oxide, but these performance gains decreased sharply at higher current densities. For PtRu, voltage gains of up to 125 mV were observed. Modification of Pt and PtRu back catalysts with Mo oxide also significantly improved their activities in direct formic acid cells, but performances in direct methanol fuel cells were decreased.