The chemical activity of organic chemicals directs their diffusion and partitioning and is consequently crucial for their transport, distribution, and toxic effects. A silicone membrane equilibrator is introduced for measuring the chemical activity of nonpolar organic chemicals in lipid-rich samples: (I) A 6 m poly(dimethylsiloxane) (PDMS) microtube (300 microm i.d., 640 microm o.d.) was placed in a sample, and a sample-PDMS equilibrium was reached within 10 min for 12 polycyclic aromatic hydrocarbons (PAHs) acting as model compounds. (II) A plug of 100 microL of methanol was pushed through the tube to equilibrate it with the PDMS and thus the sample. (III) This yielded an undiluted methanol extract that was injected into a high-performance liquid chromatograph (HPLC) with multiband fluorescence detection. Quantification limits expressed as unitless chemical activities ranged from 6 x 10(-9) to 5 x 10(-8), and relative standard deviations were from 6% to 19%. Chemical activities of PAHs in mussels from two polluted sites were measured between 10(-7) and 10(-5), and activity coefficients for PAHs in vegetable and fish oils hardly differed between oils. This method can be used for internal exposure measurements, for monitoring product safety/conformity, and process control. The method can also be applied to measure total analyte concentrations in lipid-rich samples and oils.