A study was carried out on the precipitation of heavy fractions and the isolation of asphaltenes from a petroleum sample using supercritical carbon dioxide (SC-CO2) as the antisolvent. The experiments were carried out in a laboratory supercritical fluid extraction device using a GAS (gas antisolvent) procedure at from 50 to 140 degrees C and pressure from 10 to 30 MPa. We determined the effects of temperature and pressure as well as of the addition of a hydrocarbon diluent on the yields of the isolated fractions. The elemental and microelement composition was determined as well as the structural properties of the components isolated in the SC-CO2 medium at various temperatures. An increase in temperature at pressures above 20 MPa as well as the addition of small amounts of toluene to the starting petroleum sample gave greater isolation selectivity, greater concentration of asphaltenes in the precipitated fractions, and drier solid particles. In contrast to C-7-asphaltenes, CO2-asphaltenes have lower aromaticity, polarity, and metal content. The proposed analytical method permits the isolation of asphaltenes in only a few hours, does not require large volumes of organic solvents, and yields asphaltenes in amounts sufficient for subsequent detailed study of their composition and properties.