The reactivity of prepared catalysts was evaluated by air breathing-type DMFC single cell test. The I–V curves of DMFC are shown in Fig. 7. When compared with commercial catalyst (from E-TEK), home-made Pt–Ru catalysts supported on the activated carbons had the similar performance. Among them, Pt–Ru/AC700 catalyst showed the highest performance in power density of 18 mW=cm2. Although Pt–Ru/AC800 and Pt– Ru/AC900 catalysts showed lower power density than other catalysts, the di'erences were as little as about 4 mW =cm2. Furthermore, it can be seen that the catalytic performance is directly related to the inter-particle distance. Though, for instants, Pt–Ru/AC800 and Pt–Ru/AC1000 catalysts have the same average metal size of 2:08 nm, they showed different catalytic activities. That is, closer inter-Pt distanced catalysts have better catalytic activity, as is similar to the case between Pt–Ru/AC700 and Pt–Ru/AC900 catalysts. This correlation shows that the control of inter-particle distance is more important than that of particle size on the catalyst as long as the particle size is smaller than 2:1 nm.