Two clay fractions of a Cambrian claystone from Estonia, consisting essentially of illite and 20% expandable illite-smectite. (I-S) were treated with C 12 and C 18 alkylammonium cations for K-exchange. Both the untreated and treated samples were dated by the K-Ar method. The treated clays lost several percent of their original K 2 O, with greater losses for longer-chain cations and for longer reaction time, in accordance with previously published studies. The dates of the treated clay fractions were 20-30 Ma lower than those of the untreated clays. The decrease in the dates suggests preferential opening of older, detrital clays. The K-Ar dates of the illite layers susceptible to K extraction by the various treatments were calculated by subtracting K 2 O and radiogenic 40 Ar values of the consecutive step products, and they were plotted against the total % K 2 O removed, used as an indicator of the reaction progress. Extrapolation of the plot revealed a detrital (1550 Ma) and a diagenetic (380 Ma age for the 2 illitic minerals present in the investigated shale sample. The inferred Devonian age of diagenesis of the Estonian clay corresponds to the period of massive dolomitization in the area. Both alteration processes can be related to a Devonian incursion of hot or alkaline fluids, which helps to explain the occurrence of 20% expandable I-S in claystones that have never been buried more than 1000 m. Extrapolated K-Ar ages and K 2 O contents of the illitic minerals, estimated from the X-ray diffraction (XRD) data, were used to model the experimental data. A good agreement was reached when dilution effects (chlorite and expanded illite) were taken into account.