Pristine stars with masses between \sim 140 and 260 M _ { \odot } are theoretically predicted to die as pair-instability supernovae .
These very massive progenitors could come from Pop III stars in the early universe .
We model the light curves and spectra of pair-instability supernovae over a range of masses and envelope structures .
At redshifts of reionization z \geq 6 , we calculate the rates and detectability of pair-instability and core collapse supernovae , and show that with the James Webb Space Telescope , it is possible to determine the contribution of Pop III and Pop II stars toward reionization by constraining the stellar initial mass function at that epoch using these supernovae .
We also find the rates of Type Ia supernovae , and show that they are not rare during reionization , and can be used to probe the mass function at 4-8 M _ { \odot } .
If the budget of ionizing photons was dominated by contributions from top-heavy Pop III stars , we predict that the bright end of the galaxy luminosity function will be contaminated by pair-instability supernovae .