Synthetic (co)polymers or (co)oligomers with two (or more) repeating groups show not only molar mass distribution, but also composition and sequence distribution of the individual repeat units. To characterize such two- (or more-) dimensional distribution, liquid chromatography under "critical conditions" has been suggested, where the separation according to one type of repeating units is suppressed by balancing the adsorption and the size-exclusion effects. In present work it is shown that by combination of adequately selected separation conditions in normal-phase and in reversed-phase systems, the two-dimensional distribution mode can be adjusted to result in the separation following the distribution of any of the two repeat units in ethylene oxide-propylene oxide block (co)oligomers. Based on the retention mechanism suggested, prediction and optimization of the conditions for isocratic and gradient-elution separations of (co)oligomers is possible. HPLC-MS with atmospheric-pressure chemical ionization is a valuable tool for unambiguous identification of the individual (co)oligomers and their tracking in course of method development. Gradient elution can be used for the separation and characterization of block (co)oligomers of ethylene oxide (EO) and propylene oxide (PO) according to the number of the units in one block, while the separation according to the distribution of the units in the other block is suppressed. The effects of the arrangement of the individual EO and PO blocks in the block (co)oligomers (the sequence distribution) affects significantly the retention behavior and the selection of the optimum separation conditions.