RATIONALE: SLAP is one of the two calibration materials for the isotopic water scale. By consensus the established δ 18O value is -55.5‰, although there are firm indications that δ 18O SLAP is significantly more negative. The real δ 18O SLAP value as such does not influence the isotopic water scale, however knowledge of the size of isotopic scale contraction in stable isotope measurements is vital for second order isotopes. In this study quantification of δ 18O SLAP with respect to δ 18O VSMOW is described. METHODS: SLAP-like water was quantitatively mixed with highly 18O enriched water to mimic VSMOW. The 18O concentration was determined using an electron ionization quadrupole mass spectrometer. The isotopic composition of the SLAP-like and VSMOW-like waters were measured with an optical spectrometer, alongside real VSMOW and SLAP. RESULTS: This study resulted in a much more depleted δ 18O value for SLAP than expected. The averaged outcome of 7 independent experiments is δ 18O SLAP -56.33 ± 0.03‰. There is a large discrepancy between the actual isotopic measurements of even the most carefully operating groups and the true δ 18O value. CONCLUSIONS: Although this finding as such does not influence the use of the VSMOW-SLAP scale, it raises the intriguing question what we actually measure with our instruments, and why even a fully corrected measurement can be so far off. Our result has consequences for issues like the transfer of δ 18O from and to the VPDB scale, various fractionation factors, and the Δ 17O. The absolute 18O abundance for SLAP was determined at 1.88798 (43) x 10 -3 based on the absolute 18O abundance of VSMOW and the presented δ 18O SLAP in this paper.