Adsorption–desorption isotherms for nitrogen, carbon dioxide, oxygen and helium on as-received TCM-128 ultramicroporous carbon are reported for pressures up to 60 atm at 35 °C. Evidence is presented that suggests that there are regions in the carbon that are composed of tiny hydrophobic constrictions in series, which are hardly penetrated, and more open pores inside. At room temperature, water molecules cannot penetrate these constrictions in reasonable time due to a clustering effect, while the much bigger, but unclustered nitrogen and carbon dioxide molecules do penetrate these constrictions at a measurable rate. These regions are responsible for unexpected hysteresis observed for nitrogen and carbon dioxide at 35 °C and the unusually large amount of helium adsorbed.