A new X-ray diffraction method for the determination of the amount and distribution of fixed in illite-smectite has been developed. Illite-smectite was saturated with K+ and heated at 150 °C. The 002 and 005 reflections were recorded with steps of 0.01° 2θ, and the experimental d values and the values for full-width at half-height (FWHH) were determined using a peak-profile–fitting procedure. Peak profiles were calculated with the NEWMOD program for illite structures having different amounts of and different patterns for the distribution of in interlayers. For Upper Jurassic illite-smectite from North Sea oil source rocks, the amount and the distribution of in illite interlayers were determined by comparing the experimental values for d005 and FWHH with the values calculated for the selected illite structures. The amounts of determined in this manner correlate well with the amounts determined by an isotopic dilution method. The results demonstrate that these illite-smectite samples have K end-member illite and NH4 end-member illite (tobelite) layers and that the illite layers formed during diagenesis and oil generation actually are tobelite layers.