Measurements of biexciton decays in semiconductor nanoparticles are easily contaminated by contributions from photoproducts or higher excitons. Theoretical work has shown that multiple population-period transient spectroscopy (MUPPETS) can measure biexciton decays free from these interferences. In this communication, the biexciton decay of CdSe/ZnS core-shell nanoparticles is measured with MUPPETS. The decay is strongly dispersed (nonexponential) with a more than 5-fold range of rates. This large dispersion must be accounted for in the decay mechanism and in the measurement of biexciton dynamics by more conventional methods. The success of MUPPETS in this context lays the foundation for using it to study exciton-exciton interactions in a variety of materials.