A new operationally defined method for the measurement of fixed inorganic nitrogen (N fix ) associated with clays has been tested and applied to a sequence of mixed-layer illite/smectite (I/S) (ranging from 40 to 68% illite in I/S) in shales from a drill site in Terrebonne Parish, Louisiana. This new method has distinct advantages over other methods in that it is simple and can accommodate small sample sizes (10 mg). N fix is measured, using a carbon-nitrogen analyzer, on a sample that has been ashed for 8 hr at 450 degrees C to remove organic and non-fixed inorganic nitrogen. Total nitrogen is measured on an separate untreated sample. Content of organic carbon and nitrogen can be assessed by the weight-percent difference between untreated find ashed samples. Fourier-transform infrared spectroscopy confirmed the presence of nitrogen as ammonium in untreated and heat-treated samples. N fix content of the < 0.2 mu m fraction of the shale sequence correlates positively ( R 2 = 0.91) with the percent of illite in the mixed-layer I/S. Corresponding with the increase in percent of illite in I/S with depth, an increase in N fix content is also seen with increasing depth of burial. Nitrogen from thermal breakdown of organic-matter during later diagenesis probably provides the N fix found in I/S. In the range of diagenesis studied here, N fix concentration appears to depend on the available fixing capacity of illite. Because nitrogen content varies with organic-matter type, however, caution should be used when assuming this relationship for other diagenetic environments.