The nature of Al-O bonds in K2O-Al2O3-B2O3 glasses has been studied by using the optical absorption spectra of VO2+ ions dissolved in the glasses. It has been found that the Al-O bonds in the glasses involve a double-bond character found only in the Al-O bonds in a structure unit containing a four-coordinated Al atom. The bonding mode of Al atoms in the glasses has been discussed in the light of the information obtained from the X-ray emission spectra for some aluminoborate glasses. Two modes have been suggested to be considered. In a mode, the only K+AlO4- group is formed independently of the glass composition and, in the other, the K+(K+AlO6-)- group, together with an AlO4, is formed in the low K2O region and the K+AlO4- group is dominantly formed at the high K2O content.