Transmission X-ray diffraction (XRD) of C (sub 6-16) alkylammonium ion-exchanged montmorillonite SAz-1 with Ag-behenate as an internal standard provided accurate estimates of d(001) values of the alkylammonium ion-exchanged montmorillonite. Inspection of d(001) values were made to assess the possible formation of gauche conformers (alternate arrangements of the molecules) in the interlayer and to determine the critical carbon chain length, n c . Using conventional constraints for n c (I') and n c (II) equal to 1.36 and 1.77 nm, respectively, provided ambiguous n c (I') and n c (II) values. The dependence of full-width at half-maximum values on n c allowed better estimates of n c (I') and n c (II) because "integral" and "non-integral" peak characteristics relating to interstratification could be included in the assessment. The effect of inaccurate estimates of n c (I') and n c (II) values on calculated interlayer cation exchange capacity (C i ) using two conventional concepts of calculation were compared. It was found that a procedure based on the summation of fractions of the layer charge gave relative errors of C i <2%, even where the number of carbon atoms corresponding to both n c (I') and n c (II) differed by + or -1 n c from the correct values. This method of calculation of C i is recommended when reliable values of C i are needed.