The behaviour of organic complexes of actinides and lanthanides in clays plays an important role in environmental remediation. Application of P-31 NMR to distinguish Eu(NO3)(3)-complexed tributyl phosphate from uncomplexed tributyl phosphate (TBP) facilitates an understanding of the fate of these species in the clay hectorite. Solution P-31 NMR studies show dramatic shifts of -156.0 to -172.9 ppm for Eu(NO3)(3) . 6H(2)O dissolved in TBP at TBP:Eu ratios 1:1 to 1:2.5. Pure TBP exhibits a P-31 chemical shift of -0.3 ppm. Mixtures with higher TBP:Eu ratios display lines progressively downfield of -156.0 ppm, reflecting exchange of complexed and free TBP. The Eu(NO3)(3)-complexed TBP adsorbed into hectorite displays a P-31 chemical shift of -180.7 to -193.8 ppm. Clays adsorbed with solutions that are 3:1, 5:1 and 7:1 TBP:Eu display peaks due to complex as well as peaks in the range -4 to -6 ppm attributed to uncomplexed TBP. No evidence of exchange is observed, Spectra of the TBP-adsorbed clay give a TBP line at -0.7 ppm which changes slightly to -5.0 after exchange with Eu(NO3)(3) The Eu(NO3)(3)-exchanged hectorite displays a line at -17.6 ppm after adsorption of TBP. Neither of the latter two samples show indication of complex formation. The hectorite samples exposed to TBP or the complex display (001) d spacings of 17.9-18.4 Angstrom compared to 12.3-12.5 Angstrom for hectorite which has not been exposed to TBP.