The time of emplacement of gas into each of two reservoirs in the Rotliegendes Sandstone, one in the Groningen field and one in the Broad Fourteens basin, was estimated from potassium/argon (K/Ar) ages of diagenetic illitic clays and careful petrographic observations of the rocks in which the clays occurred. In each reservoir, the intensity of diagenetic alteration increased downward within a relatively thin (15 to 175-m or 50 to 580-ft) interval, apparently reflecting a longer period of alteration for deeper samples. This is expected in cases where a gradually accumulating gas column expels pore water from progressively deeper parts of the reservoir rock. Illite is a common diagenetic phase in the Rotliegendes. Because detrital contamination in these rocks has been infer ed to be minimal, K/Ar ages of very fine-grained illite should indicate the time the illite-forming process ceases. In each case reported here, the age of the deeper sample is younger than that of the shallower. We interpret the age of the finest illite fraction in the gas zone of each reservoir to indicate the time at which illite formation ceased as the result of gas emplacement and pore fluid displacement in the rock. Our results indicate that gas emplacement in the Groningen field commenced prior to 150 Ma, and that the growth of diagenetic illite continued in the present-day water zone from that time until at least 120 Ma.