Abstract The transformation of smectite-group clay minerals to illite has garnered considerable interest as a potentially important process affecting both the mechanical and hydrologic behavior of subduction zones. Illitization can generate fluid overpressure by release of bound water, and the mineralogical change and associated cementation may increase intrinsic frictional strength while decreasing the sliding stability of faults. Released bound water also contributes to pore water freshening observed in boreholes at numerous margins. Here the authors combine data from Ocean Drilling Program drill sites along two transects at the Nankai subduction zone with numerical models of smectite transformation, to (i) quantify the distribution of smectite transformation and fluid production downdip of the trench; and (ii) evaluate its hydrologic and mechanical implications. High heat flow (ca 180 mW/m2) along the axis of the Kinan Seamount Chain (Muroto transect) initiates clay mineral transformation outboard of the trench, whereas lower heat flow (70–120 mW/m2) 100 km to the SW (Ashizuri transect) results in negligible presubduction diagenesis. As a result, considerably more bound fluid is subducted along the Ashizuri transect; simulated peak fluid sources down-dip of the trench are considerably higher than for the Muroto transect (ca 1.2–1.3 × 10−14/s vs ca 6 × 10−15/s), and are shifted ca 10 km further from the trench. More generally, sensitivity analysis illustrates that heat flow, taper angle, incoming sediment thickness, and plate convergence rate all systematically affect reaction progress and the distribution of bound water release down-dip of the trench. These shifts in the loci and volume of fluid release are important for constraining fluid flow pathways, and provide insight into the links between clay transformation and fault mechanics.