The acidities and proton affinities of hypohalous acids HOX and also hydrohalic acids HX for X = F, Cl, Br, and I are calculated through the Kohn-Sham version of spin-polarized density functional theory with several available local as well as nonlocal gradient-corrected exchange-correlation functionals. The calculated values are observed to be in good agreement with the available reported results. Unlike the proton affinity or acidity values, the calculated gross electron populations at the atomic sites are not monotonic on going from HOF to HOI and thus cannot explain the calculated proton affinity or acidity trend. However, the trends in acidity as well. as proton affinity are rationalized, in general, in terms of the calculated values of atomic Fukui reactivity indices.