We describe a differential pressure measurement device which allows one to estimate fluid pressure gradients ranging from 6.3¿10-6 to 3.15¿10-3 atm/m in soil diffuse volcanic gas emanations. The lower limit roughly corresponds to the atmospheric (static) pressure gradient due to gravity, and should represent the minimum required value for volcanic gases to reach the Earth's surface by viscous flow. In the study of diffuse gas emanations, such as for the purpose of volcanic surveillance, such physical data supplements the geochemical monitoring of concentrations and fluxes of specific substances constituting the flowing gaseous mixture and may be used to infer volcanic mechanical states. We describe laboratory experiments with the device and report on a successful preliminary test with diffuse gas emanations on the Miharayama cone of the Izu-Oshima volcano (Japan), where the highest fluid pressure gradients have been found to be associated with the highest CO2 fluxes and lowest CO2 concentration gradients. ¿ 2000 American Geophysical Union