Boronic acids are an interesting but still poorly studied class of carbonic anhydrase inhibitors. Previous investigations proved that derivatives incorporating aromatic, arylalkyl, and arylalkenyl moieties are low micromolar to millimolar inhibitors for several α- and β-CAs involved in pathologic states. Here we report a high-resolution X-ray study on two classes of boronic acids (phenyl and vinyl) in complex with hCA II. Our results unambiguously clarify the binding mode of these molecules to the human carbonic anhydrase active site, which occurs through their tetrahedral anionic form, regardless of the nature of the organic scaffold. Data here presented contribute to the understanding of the inhibition mechanism of boronic acids that can be fruitfully used for the rational design of novel and effective isozyme-specific carbonic anhydrase inhibitors.