A method is presented that makes use of nuclear magnetic resonance (NMR) measurements to differentiate between mobile and immobile pore fractions in an organic peat specimen. The method is based on the T1 relaxation mechanism and uses a paramagnetic reagent to mark the effective (mobile) pore space in the specimen under examination. A setup has been developed that allows controlled fluid flow through an NMR specimen while it is positioned in a magnet. Results from tests using fluid-filled glass tubes illustrate the application of the method and verify its accuracy. Results from a test using a peat moss specimen demonstrate the use of the method during a flow experiment in a wetland soil. ¿ 2001 American Geophysical Union