Mudrock samples from a well penetrating Eocene and Paleocene strata (1235-4455 m) of the Texas Gulf Coast have been studied to assess geochemical redistribution in mudrocks during burial diagenesis, and the mechanism by which detrital smectitic illite/smectite (I/S) is transformed into diagenetic illitic I/S. Idealized smectite and illite end-member compositions for interstratified I/S are estimated to be [K (sub 0.00) X (super +1) (sub 0.56) Mg (sub 0.39) Fe (sub 0.57) Al (sub 1.13) Si (sub 3.90) O 10 (OH) 2 ] and [K (sub 0.53) X (super +1) (sub 0.18) Mg (sub 0.17) Fe (sub 0.16) Al (sub 2.28) Si (sub 3.40) O 10 (OH) 2 ], respectively. There is no obvious difference in the amount of K addition to I/S in random- and ordered-interstratified I/S. Aluminum substitution into both tetrahedral and octahedral sites suggests that the smectite-to-illite reaction is a complete dissolution-precipitation reaction rather than a solid-state [K (super +1) + Al (super +3) ] for Si (super +4) substitution reaction. Depth-related mineral trends and mass-balance calculations suggest that illitic I/S may form from kaolinite, and possibly illite or mica, as well as from smectitic I/S. Although most whole-rock element abundances are invariant over the sampled depth interval, K 2 O content increases from ca. 2.0 weight percent at 1500 m of burial to ca. 3.8 weight percent at 4000 m. Some of this increase is probably caused by import of K into the shale via fluids derived from interbedded sandstones, implying that Gulf Coast mudrocks behave as open chemical systems during burial diagenesis. If this is the case, a minimum of 10 3 pore volumes of fluid must have passed through the most K-enriched shales to have introduced the added potassium, provided that the K content of present-day formation water is representative of that of ancient formation water.