In the chromatographic separation of macromolecules with a porous stationary phase, the retention is determined by both size exclusion and interaction mechanisms. At the chromatographic critical condition, the effects due to the two separation mechanisms compensate each other, and the retention of homopolymer molecules becomes independent of molecular weight. Liquid chromatography at the critical condition has attracted much interest for the characterization of block copolymers since it might permit the characterization of individual blocks of a block copolymer by making one block chromatographically "invisible". In this study, we critically examine this method using two sets of styrene-isoprene block copolymers designed to have one block length constant while varying the other block length. For these block copolymer systems we found that a block cannot be made completely "invisible" at the critical condition of its homopolymer, and the retention of block copolymers is affected to some extent by the length of the "invisible" block under its chromatographic critical condition.