Direct methanol fuel cells (DMFCs) are considered a clean source of electrical power for future energy demand, creating a potential to reduce our dependency on fossil fuels. Despite their advantages, including high energy density, efficiency and easy handling and distribution of fuel, the commercialization of DMFCs has suffered from some drawbacks, including methanol crossover and contamination of the system. Metal cation contaminants (such as Ni, Co, etc) introduced through the degradation of fuel cell components (bipolar plate and electro-catalyst layer) can significantly affect the Nafion-membrane properties and overall fuel cell performance. In the current study, a systematic approach is taken to characterize and identify the mechanism of the effect of metal solution contaminants on the activities of electro-catalysts of DMFCs. Cyclic voltammetry and rotating disk electrode (RDE) techniques were utilized in order to characterize the effect of various concentrations (i.e., 2x10-x M (x=1-7)) of six metal solution contaminants (i.e., Co, Ni and Zn with sulfate and nitrate as counter-anions) on the voltammetric properties and electro-catalytic activity of polycrystalline Pt during methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). The results showed a decrease in the MOR and ORR activities of Pt as the concentration of metal solution increased. The effect of counter-anion on the Pt activity was further investigated. The results showed that a combined effect of counter-anions and metal cations may be responsible for the decrease in the electro-catalytic activity of Pt. The effect of metal solution contaminants on the Nafion-ionomer of anode electro-catalysts was investigated using Nafion-coated Pt electrode. Voltammetric properties and MOR activities of Nafion-coated and bare Pt electrodes in the presence of Ni solution contaminants were characterized using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The overall results showed a significant negative effect of Ni solution contaminants on the electro-catalytic activity of bare Pt electrode as compared to the Nafion-coated Pt electrode. Based on the results, it appears that Nafion-ionomer film may interact with metal cations (through its sulfonate groups) and repel them away from the Pt active sites, partially inhibiting the negative effect of metal cations on the Pt activity of Nafion-coated Pt electrode. The effect of metal solution contaminants on the carbon-supported platinum nanoparticle (Pt/C) with various Nafion-ionomer distributions and contents (i.e., Nafion-incorporated Pt/C and Nafion-coated Pt/C electrodes) was further investigated. Cyclic voltammetry and EIS techniques were employed to characterize the effect of Ni solution contaminants on the voltammetric properties and MOR activities of Nafion-incorporated and Nafion-coated Pt/C electrodes. The overall results showed a stronger negative effect of Ni solution contaminants on the electro-catalytic activity of Nafion-incorporated Pt/C electrodes as compared to the Nafion-coated Pt/C electrodes. This further confirms previous observations showing the sulfonate groups of Nafion-ionomer film may attract the Ni metal cations, localize them away from the Pt active sites, and subsequently suppress the negative effect of cations on the activity of Nafion-coated Pt/C electrodes.