We analyse the rare kaon decays K S → γ γ and KS → γl+l− (l = e or μ) in a dispersive framework in which the weak Hamiltonian carries momentum. Our analy- sis extends predictions from lowest order SU (3)L × SU (3)R chiral perturbation theory (χ PT3 ) to fully account for effects from final-state interactions, and is free from ambiguities associated with extrapolating the kaon off-shell. Given input from KS → ππ and γγ(∗) → ππ, we solve the once- subtracted dispersion relations numerically to predict the rates for KS → γγ and KS → γl+l−. In the leptonic modes, we find sizeable corrections to the χ PT3 predictions for the integrated rates.
