Interpolymer radical coupling was investigated in polymer solutions exposed to high-intensity ultrasound. Fluorescence-detection gel permeation chromatography was used to characterize the sonication product from solutions of blends of high-molecular weight polystyrene (PS)/pyrene-labeled PS and poly(n-butyl methacrylate)/pyrene-labeled PS, showing that, along with chain scission leading to macroradical formation, sonochemically induced reactions can lead to interpolymer radical coupling and measurable block copolymer on timescales of 24 min. This investigation provides evidence of rapid in situ block copolymer formation that has previously been inferred without proof in sonication studies of polymer blends and their solutions. It is also consistent with recent studies of melt and solid-state processing of blends indicating that in situ interpolymer radical coupling is useful in reactive compatibilization. (C) 2003 Elsevier Science Ltd. All rights reserved.