Small-angle neutron scattering using contrast variation by H2O/D2O has been applied for the structural investigation of kappa -carrageenan/cetylpyridinium chloride (CPC) complexes. interaction of kappa -carrageenan with an ionic surfactant involves self-assembly of the surfactant molecules in conjunction with the carrageenan molecules and the formation of an ordered structure. Contrast matching conditions for the polysaccharide (70% H2O/30% D2O) allowed to obtain the small-angle scattering pattern from the surfactant part of the complexes. The SANS curves reveal that the observed scattering heterogeneities represent regions with sharp phase boundaries. From the Guinier part of the scattering curves, the size of these regions is determined. The obtained values are in good agreement with the structural characteristics of the kappa -carrageenan/CPC complexes determined independently by SAXS and SANS from the position and the half-width of the Braggs peaks of the scattering patterns. (C) 2001 Elsevier Science Ltd. All rights reserved.