Abstract: The stoichiometric ratio of dihydric phenol to diarylcarbonate during production of polycarbonates can be evaluated by infrared analysis, preferably using an FTIR spectrometer, to detect the amount of BPA or other dihydric phenol in the mix tank. In one format of the method, one or both of two characteristic peaks, reflecting the methyl CH bonds and hydroxyl OH bonds are evaluated. These peaks occur equally in BPA monomers and oligomers, and hence are independent of the degree of oligomerization or polymerization which has occurred. For this reason, monitoring of stoichiometry can be performed on the contents of the mix tank, or on the products from or contents of any downstream reactors. Using a calibration curve, the absorbance associated with these bonds is converted into a measure of the concentration of BPA and from that to first moles and then grams of BPA in the mixture being tested. The amount of DPC is determined by subtracting the number of grams of BPA from the total initial sample weight.