Activated carbons were prepared from carbonized phenolic resins by steamactivation. Three different novolak type phenol formaldehyde resins were synthesized under different formaldehyde (F) to phenol (P) molar ratios (F/P = 0.3, 0.5 and 0.8) and were used as the precursors for activated carbons. The activated carbons obtained from resins synthesized under typical FIP ratios were microporous, which is characteristic for activated carbons derived from phenolic resins. However, it was found that by decreasing the FIP ratio of the precursor resin to values lower than those usually adopted for commercial phenolic resin production, i.e., values lower than 0.5, activated carbons with developed mesopores could be obtained. Structural information of the precursor resins obtained from Raman spectroscopy indicated that although typical novolak resin structures could be found in all the resins, the resins synthesized at lower FIP ratios included larger amounts of phenol and lower molecular weight substances. This is thought to have led to a large difference in their thermoplastic natures. The existence of low molecular weight substances was also confirmed through thermogravimetric analysis of the resins during carbonization. CO2, C2H6, n-C4H10 and i-C4H10 adsorption isotherms of the prepared activated carbons were also measured at 298 K for the better understanding of micropore development within the samples. The obtained data suggested that micropore development was not so much influenced by the FIP ratio of the precursor resin.