The X-ray diffraction method previously developed for the determination of the amount and distribution of fixed NH4+ in illite-smectite has been modified to include the effects of layer thickness fluctuations of K-saturated and heated smectite and the effects of mean thickness of coherent scattering domains (CSDs). X-ray diffraction patterns and 002 and 005 reflection profiles are calculated for K-saturated and dehydrated NH4+-bearing I-S representing models having different distributions of K and NH4+ over mica-like interlayers, different proportions of interlayer types, different mean thicknesses of CSDs, and different degrees of thickness fluctuations for K-saturated and heated smectite layers. The diffraction criteria for identification of these models are discussed. The amount of fixed NH4+ can be determined accurately from the position of the 005 reflection. Diffraction methods have low sensitivity to different distributions of fixed K and NH4 over mica-like interlayers in NH4-bearing I-S containing a high amount of expandable interlayers and a low amount of fixed NH4. However, the interstratified nature of NH4-bearing illites or I-S can be determined unambiguously in two limited cases: first, when the structures have a low (<20%) content of expandable layers (WS) and, secondly, when NH4/(NH4 + K) ≥ 0.20 in the mica-like interlayers, even for WS > 0.20. The method is applied to I-S from western Greenland Cretaceous oil source rocks heated by intrusions. The samples contain I-T-S consisting of 13–33% tobelite layers. Two groups of samples are identified. One includes I-T-S structures in which dehydrated K-smectite layers have no significant thickness fluctuations. For these samples different broadening of 002 and 005 reflections is due only to interstratification of the 9.98 and 10.33 Å layers. In the other group a satisfactory agreement between the experimental and calculated positions and profiles of the 002 and 005 reflections is achieved only when thickness fluctuations for the K-smectite layers are taken into account.