The use of in situ Fourier Transform Infrared (FTIR) to follow PU reaction kinetics has been studied using a heated probe matching the PU material temperature. The results obtained show that, although not giving access to concentration of formed chemical bonds, the technique is a unique tool to assess subtle differences in catalysts behavior during actual PU formation, going beyond the currently used but restrictive characterization of catalysts in terms of gelling, blowing or trimerization grades. Examples of the use of the technique for the development of new amine catalysts are given for flexible and rigid foams. For flexible foams, it is shown that FTIR characterizes the gel potency in good agreement with the behavior of known gel catalyst molecules and is therefore a good tool to develop new ones. Results comparing gel strength and foam emission data show that several experimental grades have potential to be brought on the JEFFCAT® Catalyst range. They also confirmed the good performance of commercial low emission grades such as JEFFCAT® ZF-10 as blow catalyst and JEFFCAT® ZR-50, DPA and Z-130 as gel catalysts. For rigid foams, it is shown that FTIR provides good insight into the extent of trimer formation and is therefore the technique of choice to optimize catalyst blends for best fire performance and processing.