The burial-diagenetic trends of illite/smectite (US) mixed-layer clays in the Beaufort-Mackenzie Basin have been established from detailed studies of 215 core and drill-cutting samples from 22 exploratory wells onshore and offshore. Illitization in the Beaufort-Mackenzie Basin is characterized by four types of VS: random, a mixture of random and ordered, R1-ordered, and R>1-ordered US. In general, the proportion of I-layers increases with depth, while the dominant type of I/S changes from random to R>1-ordered I/S. The increase of I-content is fast and shows a parabolic trend with respect to depth in the interval of random US, whereas it is slow and linear in the range of ordered I/S. Temperature appears to be the important control for the illitization reaction, while geopressure, lithology, and other geological variables contribute substantially to local variations in I/S composition. A temperature of more than 50 degrees C is the threshold temperature for activation of the illitization reaction. The transitions from random to R1, and from RI to R>1-ordered VS occur at 70 degrees-110 degrees C and 110 degrees-140 degrees C, respectively. Illitization in geopressure zones and sandy intervals appears to be more advanced than in normally pressured sections and shales, respectively. Accelerated VS diagenesis in geopressured environments is probably related to higher water/rock ratios compared to normally pressured conditions. The same reasoning may apply to sandy sections because of their greater porosity and permeability. Hydrocarbon occurrence in the Beaufort-Mackenzie Basin appears to be related to I/S diagenesis. Hydrocarbon reservoirs are located above or in the top portion of the active illitization zone within the interval of random I/S interval, but rarely below the depth where ordered I/S appears. Smectite-water derived from I/S diagenesis may have played a role in the emplacement of hydrocarbons.