Serpentinization ubiquitously affects ultramafic rocks that interact with water, with strong implications for the origin of ancient microbial ecosystems, the chemical budget of the ocean, and the rheology of the oceanic crust. The increase in volume associated with this reaction, and its consequences for reaction progress, have been debated for over a century. Serpentine minerals are ~40% more voluminous than olivine, which suggests that fully serpentinized peridotite should have negative, or at least very low, porosity. Recent studies have proposed that self-propagating, reaction-driven fracturing can facilitate serpentinization reactions, but the nanoscale mechanisms by which fluid is transported through serpentinized fractures in order to react with fresh olivine surfaces remain poorly understood. To address this issue, we studied a sample of serpentinized harzburgite collected during ODP expedition 209 at site 1274, using Focused Ion Beam – Scanning Electron Microscopy (FIB-SEM)-assisted tomography. We specifically targeted the interface between the serpentinized peridotite and the unaltered primary mineralogy. The resultant images illustrate the presence of nanopores within the serpentine alteration products, primarily at the interface with the unreplaced minerals. Importantly, no pores were observed in the serpentine away from the grain boundaries with olivine, suggesting that these nanopores form during the initial stage of reaction, and then disappear with further reaction progress. We argue that the observed nanoporosity is an intrinsic feature of the serpentinization reaction, and that its transient presence during serpentinization is vital for facilitating reaction progress. We suggest that the transient nature of the pores arises from the opposing kinetics and thermodynamics of the replacement reaction. The former promotes the formation of voids that enhance the advective transport of fluids to the reaction front, while the latter drives the reduction of pore space by means of recrystallization of the serpentine aggregates and minimization of the interfacial energy.