The ages of diagenetic and detrital illite are discordant in clastic argillaceous rocks such as shales and paleosols. This study attempts to separate or remove the detrital illite component using sodium tetraphenylborate (NaBPh4). The three clay fractions (<1 µm, 1-2 µm, and 2-5 µm) from three well-studied Illinois Basin paleosols were exchanged with NaBPh4 at room temperature for various durations: 1-hour, 1-day, and 22-days. The NaBPh4 treatment decreased the intensity of the 10 Å diffraction peak for detrital illite and/or shifted it to higher d-spacings values. NaBPh4 did not change the mineralogy of the illite-smectite of the potassium bentonite clay fraction. The K-Ar age values decreased as much as 50 Ma in the coarse clay after the 22-day treatment. The associated K-Ar ages values for illite-smectite decreased only slightly greater than the measured uncertainties for the I-S. This treatment shows promise for removing detrital illite from clay fractions.