Two kinds of mudstones from the Tarim Basin and the Tuha Basin were treated with C8 and C18 alkylammonium cations. Both of them were dated by the K-Ar method. One kind of rock contains mainly diagenetic illite, and the other contains mixed layer illite/smectite (I/S). Sample separation and alkylammonium cations treatment were performed first, and K-Ar dating followed. The result shows that fine-grained minerals have younger K-Ar ages and coarse-grained components have older ages. Plots of K-Ar age versus K2O (%) are linear, the range of diagenetic age can be estimated by extrapolation. The reaction results of the above samples are different after alkylammonium cations treatments. The samples containing mainly illite show a decrease in K2O and an increase in age by 1-25 Ma, suggesting that preferential exchange of young diagenetic clay rather than detrital clay by alkylammonium cations, therefore the age of diagenetic illite can be calculated directly by the % K2O and radiogenic 40Ar removed. For the I/S dominant samples, detrital illite was opened preferentially by C8 alkylammonium cations treatment. Detrital age can be calculated, but it is hard to obtain the diagentic age directly.