This document specifies methods for the measurement of toluene-2,6-diisocyanate in mixtures of the 2,4- and 2,6-isomers. Two methods based on infrared spectroscopy are used to give accurate results over a broad range of isomer concentrations. Method A is applicable to toluenediisocyanate (TDI) samples containing between 5 % and 95 % of the 2,6-isomer. Method B is applicable to TDI samples containing 0 % to 5 % of the 2,6-isomer. Both methods are based on the quantitative measurement of absorption bands arising from out-of-plane C−H deformation vibrations of the aromatic ring at 810 cm⁻¹ and 782 cm⁻¹ (12,3 μm and 13,8 μm).