Two mono-substituted EDO-TTF derivatives, 4,5-ethylenedioxy-4 '-methyl-TTF (MeEDO-TTF, 2) and 4,5-ethylenedioxy-4 '-methylthio-TTF (MeSEDO-TTF, 3), were synthesized in the moderate yields. These two donors showed similar electronic structures of the pi-electron systems to that of the parent molecule EDO-TTF, according to the cyclic voltammetry and UV-vis spectroscopy. Despite their different compositions, the radical cation salts of these two donors exhibited similar optical spectra with a low-lying absorption band at around 3.3 x 10(3) cm(-1), which was consistent with their good conductivities at room temperature. The transport properties of these salts ranged from stable metals, metal-insulator transition systems, to semiconductors. Most of the radical cation salts showed Pauli paramagnetic behavior at around room temperature. The crystal structures of the stable metallic salts, (MeEDO-TTF)(2)X (X = BF4, ClO4), exhibited a novel donor packing pattern, and the calculated band structures showed two-dimensional feature, consistent with their transport properties.