Abstract: The matrix of the Pliocene volcaniclastics from the Akrotiri area of the Santorini island (Greece) is dominated by clinoptilolite. Smectite, occasionally illite-smectite, opal-CT, cristobalite and mordenite are also present. The clinoptilolite-rich samples were heated at 460 and 560°C for 12 h and the reductions in the intensity of the 020 diffraction peak were measured. Electron microprobe analysis (EMPA) was then used to study the chemical composition of the clinoptilolite. Statistical analysis proved a strong and quantifiable relationship between the reduction of the 020 diffraction peak of the clinoptilolite and the Na/K ratio. A representative set of microprobe analyses of clinoptilolite was performed before any correlation with thermal behavior was attempted. The presence of K in the structure of clinoptilolite as well as its relationship with Na are most important in the thermal behavior of clinoptilolite.