The isomeric composition of airborne toluene diisocyanate has been determined in two plants producing slab stock flexible polyurethane foam. The high performance liquid chromatographic technique used for analysis of the collected samples was optimized for the quantitative and qualitative determination of 2,4 and 2,6-TDI. The data indicates that there is a very large increase in the amount of airborne 2,6-TDI relative to the 2,4 isomer, as compared to the starting material used in the process. The magnitude of the increase was dependent on the stage of production. These results are consistent with the hypothesis of increased offgassing of 2,6-TDI, due to its lower reactivity.