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Online comprehensive two-dimensional liquid chromatography (LC × LC) is a technique of great importance, because it offers much higher peak capacities than separations in a single dimension. When analyzing polymer samples, LC × LC can provide detailed information on two mutually dependent polymer distributions. Because both molecular-weight distributions and chemical-composition distributions are typically present in synthetic copolymers, combinations of interactive LC with size-exclusion chromatography (SEC) are especially useful for (co)polymer analyses. Commonly applied SEC separations in the second dimension take several minutes, so that a total LC × LC experiment typically requires several hours. This renders LC × LC unsuitable for routine analysis. In the present study we have explored possibilities to perform fast and efficient online comprehensive two-dimensional analysis of polymers using contemporary ultrahigh-pressure liquid chromatography in both dimensions (UHPLC × UHPLC). Gradient-elution UHPLC in the first dimension allowed efficient separations of polymers based on their chemical composition. SEC at ultrahigh-pressure conditions in the second dimension offered very fast, yet efficient separations based on molecular size. The demonstrated UHPLC × UHPLC separations of industrial polymers could be performed within 1 h and provided comprehensive information on two-dimensional distributions.