A pilot project has been launched under the auspices of the Consultative Committee for Amount of Substance (CCQM) to evaluate the measurement issues for the amount of SiO@sub 2@ on (100) and (111) Si in the thickness range 1.5 nm to 8 nm. Wafers and methodologies have been carefully prepared to achieve the best results possible. 44 sets of measurements have been made in different laboratories using 9 different methods (MEIS, NRA, RBS, SIMS, XPS, ellipsometry, GIXRR, NR and TEM). The results have been assessed, against NPL XPS data, using d(respondee) = m d(NPL) + c. All show excellent linearity, except 3 sets with methods more suited to composition depth profiles. The main sets correlate with the NPL data with average rms scatters of 0.15 nm with half being better than 0.1 nm. Each set allows the relative scaling constant, m, and the zero thickness offset, c to be determined. Each method has 0 < c < 1 nm and it is these offsets, measured here for the first time, that have caused many problems in the past. Each technique has a different accuracy for m and consistent results have been achieved. XPS has poor accuracy for m but a high precision and, critically, has zero offset if used correctly. Achieving a consistent scaling constant and zero offset for XPS requires reference conditions and is not trivial. Analysts using these conditions generated dramatically improved data. A combination of XPS and another method allows an accurate determination of the XPS scaling constant. XPS then has a high accuracy, traceable via the other method. Several methods have small offsets which, if they can be controlled, will enable these methods also to show high accuracy.