Geochemical—and perhaps biochemical—processes may yield tell-tale proxies in rocks and minerals on the Earth or other planetary bodies, in the form of distinctive slopes of linear fractionation arrays on the oxygen three-isotope plot. It is generally recognized that kinetic and equilibrium fractionation processes are described by different mass fractionation laws. We show that coupled laser fluorination, dual-inlet IRMS procedures for oxygen three-isotope analysis of silicates, at high precision, gave reproducible accuracy for the slope value as measured independently by two different laboratories, using replicates of the same silicate samples. As far as we are aware, this is the first such inter-laboratory comparison. Hydrothermal quartz (together with one chalk flint sample) with a range in δ18O of 31‰ gave respective slope λ values of 0.5240 ± 0.0010 and 0.5242 ± 0.0010, using Prism III and MAT 253 mass spectrometers, respectively, at the Open University (OU). Errors were computed from weighted linear regression and are reported at the 95% confidence level. The comparable result obtained at the Geophysical Laboratory (GL), Carnegie Institution of Washington, was 0.5240 ± 0.0015. A MAT 252 mass spectrometer was used for the latter measurements and the oxygen extraction and purification procedures differed in detail from those used at the OU. In contrast, slopes measured for replicates of seven garnet samples, metamorphosed under high-temperature, high-pressure conditions, and spanning 20‰ in δ18O, gave 0.5262 ± 0.0008 at OU (Prism III analyses) and 0.5266 ± 0.0012 at GL.