Pt/CNafion((R)) composite electrodes prepared by a hot-pressed method were modified by chromium (Cr) for sensing hydrogen and oxygen in this study. The results indicate that the sensitivities of the Cr-modified electrodes in sensing hydrogen and oxygen rise markedly from 0.716 to 0.771 and from 0.414 to 0.533 muA per ppm, respectively, and the response and the recovery times on the Cr-modified electrode are shortened. Meanwhile, the Cr-modified Pt/C/Nafion((R)) electrode can depress the aging in continuously sensing oxygen for 250 h. After aging, the sensitivities of the Pt/C/Nafion((R)) electrode and the Cr-modified Pt/C/Nafton8 electrode decrease by 81 and 65%, respectively. Generally, both the response and the recovery times decrease significantly with increasing gases concentrations. A sensing model was also proposed to illustrate the sensing phenomenon.