The acid dissociation constant (pK(a)) of inorganic oxoacids was calculated using density functional theory (DFT) with the hybrid cluster-continuum approach. The results showed that calculated pK(a) values had a good correlation with the experimental values. Based on the correlation of the experimental values with the calculated ones, the pK(a1) value estimated for selenic acid, which has not been determined experimentally, agreed well with the value estimated by thermodynamic calculations.