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A novel concept for the diagnosis of a pure target gas (TG) in air is presented. The method employs a resistive gas sensor with a mm-thick highly porous electroceramic gas- sensitive body. The diagnosis is based on the fact that the diffusion time of the TG through the porous body would depend on its molecular properties. The transient response of the device was analyzed based on a diffusion- reaction equation. The solutions of the equation were defined by three parameters, which were shown to be of diagnostic merits. Such a device was fabricated by partial sintenng of a ZnO body formed by low- pressure pressing. The transient responses of the device to methanol, ethanol, poropanol and butanol were recorded. The analytical solutions were fitted to the experimental data, and the diagnostic parameters were resulted as the fitting parameters. Comparison of the parameters obtained with those available from the calibration experiments afforded TG identification.