We report on the Sr isotopic composition of pore fluids recovered during Integrated Ocean Drilling Program Costa Rica Seismogenesis Project Expeditions 334 and 344. Pore fluid samples were acidified and loaded directly onto columns containing EICHROM Sr-Spec resin, followed by analyses using a NU multicollector inductively coupled plasma-mass spectrometer at the Keck Collaboratory, Oregon State University. Sites drilled on the upper plate exhibited a decrease in the <sup>87</sup>Sr/<sup>86</sup>Sr ratio to values as low as 0.70762 in the upper 200-300 m of the sediment column, suggesting the role of ash alteration. The unconformity between the slope sediment sequence and underlying framework rock, sampled at Sites U1378-U1380, is characterized by intense fracturing. The fluids sampled within the unconformity at these sites have <sup>87</sup>Sr/<sup>86</sup>Sr ratios ranging from 0.70834 to 0.70844. Fluids sampled from the incoming plate sediment show evidence of ash alteration and carbonate diagenesis, but the deepest sediment here may also be modified by diffusion of fluids from the underlying oceanic crust. Intense carbonate recrystallization in the lower 66 m of Site U1414 results in Sr concentrations that are almost an order of magnitude higher than those measured at the base of Site U1381. The deepest pore fluid sample measured at both Sites U1381 and U1414 has a <sup>87</sup>Sr/<sup>86</sup>Sr ratio of 0.7086. The strontium systematics in this margin, used in the context of other ongoing studies, may be used to unravel processes at this margin, such as carbonate diagenesis, ash alteration, fluid migration, and diffusive exchange with the underlying oceanic crust.