Calcite shells of foraminifera, which are accumulated in the ocean sediment, are an important object of paleoceanographic studies to reconstruct environmental parameters of the past. Foraminifera are unicellular organisms living in almost all parts of the ocean during the entire paleoceanographic time scale. The isotope ratio of boron incorporated in the calcite shell delivers information about the pH-value of the ocean at the time the shell was formed. Since the boron fraction of such a shell is about 5 ppm, an extremely sensitive technique is necessary for an exact boron isotope ratio determination. Resonant laser secondary neutral mass spectrometry (r-laser-SNMS) was used to measure boron isotope ratios in calcite shells. Analysis was carried out with a time-of-flight mass spectrometer equipped with an electron impact gun for sputtering and a Ga+ primary ion source. Resonant ionization of sputtered boron neutrals was performed via a three-step ionization scheme accomplished with two tunable dye lasers and the fundamental wavelength of a Nd:YAG laser. After optimizing the boron ionization and detection process, boron isotope ratios were directly measured on single foraminiferal shells after removing contaminants by Ar+ ion beam sputtering. (c) 2006 Elsevier B.V. All rights reserved.