Di-isopropyl methylphosphonate (DIMP) and dimethyl methylphosphonate (DMMP), which are manufacturing by-products of. and surrogate compounds for, the nerve agents Sarin (GB) and VX, respectively, are readily quantitated at microgram per liter concentrations in contaminated groundwaters. Aqueous samples (typically 1 L) are first fortified with triethylphosphate (TEP) as a surrogate, then passed through a “sandwiched” set of three preconditioned extraction disks consisting of the following (in filtration order): (a) glass fiber filter, to remove unwanted particulate matter; (b) C18-based extraction disk, to collect DIMP; and (c) carbon-based extraction disk, to collect DMMP. The glass fiber filter is discarded; the two extraction disks are dried and extracted with a small volume of methanol. After the extract is fortified with diethyl ethylphosphonate (DEEP) internal standard, it is analyzed using a gas chromatograph equipped with a nitrogen-phosphorus detector (NPD). Quantitation of DMMP, DIMP, and TEP is performed using the method of internal standards.