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We report laboratory measurements of thermal conductivity and thermal diffusivity and calculated values of volumetric heat capacity for 56 core samples collected during International Ocean Discovery Program Expedition 375 from Sites U1518 and U1519 in the Hikurangi subduction zone. These sites are instrumented with borehole observatories that include downhole temperature sensors, enabling eventual integration of laboratory-derived thermal properties with in situ thermal data. Measurements were conducted under saturated conditions using a transient plane source technique and include repeated tests for quality control. Volumetric heat capacity was calculated as the ratio of thermal conductivity to thermal diffusivity, using measurements obtained simultaneously on the same sample. At Site U1518, thermal diffusivity averages 5.055 ± 0.610 × 10−7 m2/s (± one standard deviation) and volumetric heat capacity averages 2.588 ± 0.277 MJ/(m3·K). At Site U1519, thermal diffusivity averages 5.395 ± 1.027 × 10−7 m2/s and volumetric heat capacity averages 2.574 ± 0.350 MJ/(m3·K). Measured thermal conductivity values average 1.294 ± 0.123 W/(m·K) at Site U1518 and 1.354 ± 0.131 W/(m·K) at Site U1519 and are consistent with previous shipboard results. These new constraints on thermal properties provide key input for interpreting borehole temperature records and modeling transient heat transport in subduction zone fault systems.