Natural illites contain a small amount of boron which probably substitutes for silicon in the illite lattice. The exact amount of boron taken into the structure is determined by the potassium content and grain size of the illite and the salinity at the time of deposition (paleosalinity). In illites of constant grain size from the same salinity environment there is an antipathetic relationship between boron and potassium. Departure curves have been constructed from the experimental data which are used to correct for variations in boron concentration due to changes in composition and grain size of illites. The residual variations in boron concentration are believed to be directly related to paleosalinity. Thus boron content of illites can be used as a guide to paleosalinity and may be a sufficiently sensitive index of salinity to detect small changes across a subsurface reef or sand bar.