Raised bogs are important ecohydrological systems in which there are strong two-way links between plant succession, litter and peat decay, and hydrological functioning. Using recently established protocols, we measured the hydraulic structure of a raised bog in West Wales. We tested two hypotheses: (i) that the hydraulic conductivity (K) of the peat shows depth dependency such that lower layers of peat are effectively impermeable, and (ii) that the K of the marginal peat of the bog dome is lower than that in central areas. From 107 piezometer measurements we found there was depth dependency of K but that lower peat layers were not poorly permeable or impermeable. We also found that the K of the peat on the margin of the bog dome was generally significantly lower than that in central areas. Our results suggest that, for some bogs at least, it is important to simulate water flow through deeper peats when simulating peatland development or growth. They also raise the intriguing possibility that the low K of marginal peat is important in maintaining wet conditions in central bog areas, allowing bogs to reach greater thicknesses than they would do in the absence of the low-K margin; an idea first proposed for blanket bogs by Lapen et al. (2005).