The behaviour of PMMA standards in mixtures composed of two binary critical eluents was investigated. It was found that combinations of two critical eluents do not result in a molar mass independent elution (critical conditions). When mixing two binary critical eluents, elution might occur in SEC, LAC or close to critical conditions in a non-predictable fashion. It can be assumed that the deviation from critical conditions is caused by preferential adsorption of some eluent components to the stationary phase or the polymer. In order to have a fast estimate, in which mode of chromatography elution will occur when mixing two critical eluents, ternary critical conditions were estimated from gradient experiments. A good agreement was observed between the so obtained critical lines and the elution behaviour resulting in mixtures of two critical eluents.
