The structure of an illite-smectite (I-S) sample (HSI) from the Tertiary–Cretaceous boundary layer at Stevns, Denmark, i.e. the so-called Fish Clay, is investigated in order to discuss the origin of this clay and its similarity to clays from Maastrichtian-Danian chalk of Denmark and the North Sea. The phase compositions and layer sequences have been determined by X-ray diffraction. The structural formulae are determined from chemical analysis and solid-state 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The octahedral vacancy pattern has been determined by thermal analysis and the particle shape by atomic force microscopy. The HSI sample I-S consists of two phases, a high-smectitic (HS) phase (70%) having 95% smectite and 5% illite (leucophyllite) layers, and a low-smectitic (LS) phase (30%) having 50% smectite and 50% illite (leucophyllite) layers. Both phases have trans-vacant dioctahedral sheets and contain only traces of IVAl, the charge of the illite (leucophyllite) layers being provided predominantly by octahedral Mg for Al substitution. The structure of the tetrahedral and octahedral sheets for the HSI are compared to the structure of an I-S sample (GRI) from the Maastrichtian chalk at Stevns below the Tertiary-Cretaceous boundary and to the standard smectite SAz-1 from Arizona. For all three samples the 27Al MAS NMR spectra show the presence of two resolved IVAl resonances, which indicate the presence of two different IVAl sites. The NMR, infrared spectroscopy and chemical data show that both HSI and GRI have highly ordered Mg for Al substitution in the octahedral sheet, each Al having one Mg and two Al neighbours, whereas substitution of Mg for Al in SAz-1 is random. For the HSI sample, both the HS and LS phases are probably formed from volcanic ash. The structural similarity of the phases in HSI and GRI shows that GRI formed from a material similar to HSI by illitization of the HS phase and chloritization of the LS phase.