In order to obtain an insight into structural features of mesocarbon microbeads (MCMB), we examined its butylation reaction promoted by potassium or dibutylzinc. Both reactions gave THF-soluble products in 86-95% yield. This is the first example for effective solubilization of these carbonaceous materials. The resulting products were analyzed by C-13 NMR and gel permeation chromatography. The results of structural analyses indicate that MCMB molecules consist of highly condensed aromatic hydrocarbons (average ring size is calculated to be 15-16) with very minor amounts of alkyl substituents and oxygen-functional groups.