Aqueous solubilities of lower diamondoid hydrocarbons, adamantane (C10H16) and diamantane (C14H20), were measured along the 5 MPa isobar at temperatures between 313 K and the solid-liquid-vapor triplepoint temperature of the solute. The activity coefficients of the individual diamondoids in saturated aqueous solutions were estimated from the solubility data employing two different approximations of the pure-solute heat capacity difference Delta C-P2. The aqueous solubilities and activity coefficients of diamondoids were compared with those of polycyclic aromatic hydrocarbons with the same carbon numbers. Depending on temperature and on the approximation of Delta C-P2 the activity coefficient of adamantane in saturated aqueous solution exceeds that of naphthalene by a factor ranging within 70 to 150, whereas the activity coefficient of diamantane in saturated aqueous solution exceeds that of anthracene by a factor ranging within 10 to 50.