Heulandite-group zeolites are abundant in the Miocene pyroclastics from Western Anatolia, Turkey. We investigated the relation between the I(111)/I((3) over bar 11) intensity ratios measured by X-ray diffraction (XRD) and the content of exchangeable cations for 15 samples of natural heulandite-group minerals gathered from the Gordes and Bigadic regions of Western Anatolia. The intensity ratios range from 0.77 to 0.94 in natural heulandites and from 1.38 to 1.80 in natural clinoptilolites. The data obtained from Na-, K- and Ca-exchanged forms of a heulandite and clinoptilolite show that the intensity ratio increases with Na-, K- and Ca-exchange in heulandite and also with Na- and K-exchange in clinoptilolite, whereas it decreases with Ca-exchange in clinoptilolite. The intensity ratios were calculated using the known structural data of clinoptilolites to understand the effect of positions, amounts and kinds of exchangeable cations and water molecules. An increase in Na and Ca may increase or decrease the intensity ratio, depending on their sites and occupancies. Potassium causes a significant increase in the intensity ratio and an increase in Mg decreases the intensity ratio. There is a strong correlation between the intensity ratio I(111)/I((3) over bar 11) and (Na + K)/(Ca + Mg) ratio and thermal stability, both of which have been used to characterize heulandite-group minerals.