We report major and trace element and Sr-Nd-Pb isotope compositions of basement rocks recovered during Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344 (Costa Rica Seismogenesis Project [CRISP]). The major element oxide SiO2 was analyzed by X-ray fluorescence; all other major and trace element concentrations were determined by inductively coupled plasma-optical emission spectrometry and inductively coupled plasma-mass spectrometry, respectively. Sr, Nd, and Pb isotope ratios were analyzed by multicollector inductively coupled plasma-mass spectrometry. SiO2 measured at the Number 4 Exploration Institute of Geology and Mineral Resources of Shandong Province (China), and all other analytical work was carried out at Key Laboratory of Marine Sedimentary and Environmental Geology, State Oceanic Administration (China). The analyzed samples are all tholeiitic basalts that are enriched in light rare earth elements with no Eu anomalies. Most basement samples from Expedition 334 Site U1381 and Expedition 344 Site U1414 resemble enriched mid-ocean-ridge basalt. In addition, a few Hole U1414A basement samples show a positive Pb anomaly. Radiogenic isotopic (Sr, Nd, and Pb) compositions are similar to previous studies, reflecting the complexity of mantle sources and tectonic evolution of the Cocos Ridge. The trace element and Sr-Nd-Pb isotopic compositions for the recovered rocks may be further used to explain the relationship between subduction input and arc output in the subduction factory, such as the nature and role of subducted components, element cycling during subduction processing, and mantle geochemistry.