Using a primary mobile phase of supercritical carbon dioxide, the effects of various organic compounds as modifiers are studied, especially with respect to retention characteristics, selectivities, and peak shapes of various test solutes. These solutes include polynuclear aromatic hydrocarbons (PAHs), aromatic amines, alkylbenzenes, chlorobenzenes, dialkyl phthalates, and azo (-containing) compounds which are separated on commercially available packed liquid chromatographic columns. Various modifiers with different functionalities up to 20 wt. % are used to separate large PAHs with very short retention times. Formic acid is also used as a modifier to improve hydrocarbon group separations in conjunction with flame ionization detection.