Liquid adsorption chromatography at critical conditions (LACCC) in normal and in reversed-phase modes provides independent information on the molar mass distributions of both blocks of poly(methyl methacrylate)-block-poly(tert-butyl methacrylate), even if no information about the precursor is available. In addition, the amount of unreacted precursor can be determined, even if its molar mass is comparable to that of the total block copolymer. The reversal of elution order by changing of stationary and mobile phases makes it possible to independently characterize each block in the SEC mode of the LACCC system. Thus, complete structural information is obtained even without using two-dimensional chromatographic techniques.