In this work we take advantage of halide exchange (HE) at cesium lead iodide (CsPbI3) nanoparticles to develop an assay that is sensitive to bromide ions (Br-) released during an organohalide reaction. Upon sampling the reaction, the free Br- react quickly with the CsPbI3, resulting in CsPbBrxI3-x intermediates, each of which have unique blue-shifted photoluminescence (PL) wavelengths, the change of which (), we use as a colorimetric proxy of the reaction. An assay response factor, C, relates as a function of CsPbI3 concentration, as well as Br- concentration. The observed kinetics, as well as calculated rate constants, equilibrium, and activation energy of the solvolysis reaction tested correspond closely to synthetic literature values, validating the assay. Factors that influence the sensitivity and performance of the assay, such as CsPbI3 size and morphology, and concentration, are discussed.