The paramagnetic clay mineral illite can have important controls on fluid permeability and microporosity in sedimentary rocks. Increases in illite content of a few percent can reduce permeability by several orders of magnitude. Traditional X-ray diffraction (XRD) techniques for quantifying illite content can be very time consuming, requiring significant sample preparation, and generally examine only a relatively small sample volume. In contrast, a technique based on magnetic susceptibility described here is very rapid, cheap, sensitive, nondestructive, and requires no extra preparation of the sample. It is representative of a much larger sample volume, and in the present paper utilizes standard 1 inch diameter cylindrical core plugs, but it can be applied to other sample shapes and volumes. The magnetic-susceptibility-derived estimates of illite content from core plugs correspond well with XRD results from small powdered samples of the same material, in terms of both absolute values and overall trends with depth. However, neither the magnetic method nor XRD should be regarded as definitive. Each technique has merits and limitations, and these help to explain some observed differences in the illite determinations by each method. XRD may underestimate the illite content, particularly in muddy rocks, whereas the magnetic method theoretically provides an upper limit for the illite content. The magnetic method is also capable of simultaneously estimating the quartz content in the present samples. Changes in the concentration and distribution of illite during core cleaning or core flooding experiments can also be potentially quantified by the magnetic technique. The development of portable field sensors potentially allows the magnetic method to provide high-resolution illite profiles on slabbed cores, outcrops, and unconsolidated samples without the need to cut core plugs. The method could also be applied to whole-core measurements and to downhole magnetic susceptibility data. The processing of the magnetic susceptibility signal can also be extended to quantify other minerals in simple systems. © 2004.