The acid-base behavior of a colombian bentonite from the Cauca Valley was studied by potentiometric titrarion. Back-titration was used because continuous titration does not give a unique curve. The titration curve shows three inflection points at pH 4.8, 6.2, and 6.8 determined by means of the derivate. They indicate three types of acid sities. The first and strongest acid site is manifested as a substitution mechanism of the interlayer ions of the 2: 1 structure by H+. The second and third are a consequence of the broken bonds or defects of the crystal lattice in edges that originate surface complexing srnes of silanol (=SiOH) and aluminol (=A1OH) types.