Nine potassium-argon dating examples oi diagenetic clays allow us to present different types of isotopic evolution. in each study the mineralogical data help to define the nature oi analyzed clays and the isotopic data are systematically processed through the two different types oi K/Ar isotopic evolution diagrams. The sequences studied are: porous sandstones, sometimes exploited for hydrocarbons, bituminous shales or impermeable sandstones through diagenetic crystallizations. The examples showing only one instantaneous event are quite seldom. More generally, one or more ages can be calculated. The simple process corresponding to an instantaneous isotopic closure seems very often disturbed. The main disturbing mechanisms are : 1) inheritance of detrital clays showing too old and scattered ages for the studied stratigraphic level; 2) translation of isotopic lines through potassium gain and/or radiogenic argon loss; 3) determination of excess argon in some instances where confinement appears possible, and 4) slow crystallization of the studied clays which leads to an average ＇＇age＇＇. Three methodologic rules come out of these examples and those of the literature. Firstly, the most precise mineralogical studies are definitely necessary to define the implied mineralogical type(s). This is especially essential regarding the process used for the K/Ar dating of diagenetic clays, which shows the evolution from smectite to illite, by I-S. Secondly, publishing all isotopic K/Ar data will be necessary for any reprocessing, in order to reveal other interpretative models in the future. Finally, it seems very dangerous in the light of present examples, to restrict the dating of diagenesis to only one sample, even ii that sample is very carefully selected. The isotopic treatment through the evolution diagrams often define unexpected and highly interesting results; this obviously requires the analysis of more than one sample.