This dataset comprises stable silicon isotope measurements of radiolarian tests obtained using MC-ICP-MS (mass spectrometer, Thermo scientific, Neptune). Radiolarians were separated from sediments broadly following standard techniques. Sediment samples were repeatedly washed with deionized water over a 63 µm sieve. After drying at 50 ˚C, radiolarians were handpicked from the >63 µm size fraction. Siliceous microfossils were subsequently cleaned to remove clays and organic matter, digested, and subjected to column chromotography for silicon purification prior to isotopic analysis. This dataset evaluates isotopic variability among spumellarian morphotypes (1, 2, and 3; the first morphotype is distinguished by smooth edges and finer pores, the second by protruding spines and third one by bigger pores with rough edges); across size fraction (300 µm) of tests obtained from spumellarian morphotypes and compares spumellarians with nassellarians at sites IODP 371-U1511 and ODP 207-1260. IODP Site 371-U1511 (South Pacific) was drilled on the Tasman abyssal plain, ODP Site 171-1260 (Western Equatorial Atlantic) is located at the Northwest margin of Demerara Rise.