Localization of styrene-butadiene block copolymers (BC) in both quenched and slowly cooled polystyrene (PS)/polybutadiene (PB) (4/1) blends was studied by means of small-angle X-ray scattering and transmission electron microscopy. Two types of BCs were used: (a) molecular weight of the styrene blocks is 10 000, i.e. below the critical molecular weight of styrene necessary for the entanglement formation (-18 000), (b) molecular weight of the styrene blocks is 40 000, i.e. it sufficiently exceeds this value. It has been shown that both types of the BCs used are in quickly cooled samples moleculary dispersed at the interface with an exception of 40S-60B-40S-60B-40S. In slowly cooled samples with addition of BCs with "short" styrene blocks, these BCs are released from the PS/PB interface, in contrast to the BCs having "long" styrene blocks.