Fixed nitrogen in illite-smectites (I-S) has been measured for Miocene shales from a Gulf of Mexico oil well. Fixed N values for the <0.2 mu m fraction increase with depth from 150 ppm (1000 m) to a maximum of 360 ppm (3841 m). This increase is coincident with illitization from 41% I in I-S to 75% I in I-S. Below 3841 m, fixed N values decrease to 190 ppm (4116 m) while I-S is maintained with a slight increase from 77 to 82%. The changes in fixed N with increasing illitization are consistent with the notion that illitization proceeds via both transformation and dissolution/precipitation reaction mechanisms. The trend of decreasing fixed N in illitic I-S is compatible with surface-controlled crystal growth and Ostwald ripening mechanisms for illitization. The trend may also be linked to the timing of maximum NH (super +) 4 release from kerogen maturation during oil generation. The changing rate of NH (super +) 4 liberation from organic matter and multiple illitization reaction mechanisms can result in complex N geochemical cycling pathways throughout early diagenesis to metamorphism.