The volumetric water content (θ) of peat soils below the water table is largely controlled by the production of biogenic methane-rich gas bubbles that are subsequently released to the atmosphere, thereby having significant implications for carbon cycling. Geophysical methods have recently shown promise for improving studies of gas storage and release in peatlands. We investigated the relationship between dielectric permittivity and volumetric water content in organic peat soil using ground-penetrating radar. We developed a novel approach for controlling water content using a pressurized test chamber to reduce the volume of bubbles under high pressure as described by the ideal gas law. This method simulates the bubble-rich natural conditions much more closely than previous studies that utilized drying to vary water content. Our results cover a range of highly saturated peat that is commonly observed in poorly decomposed near-surface peat and we demonstrated that a linear model can be used to estimate water content in peat for a range of water contents (i.e. θ>90%). The data collected from samples taken from different peatlands suggests that it is possible to use our resulting model to convert dielectric permittivity extracted from ground-penetrating radar data into free-phase gas concentration via the water content.