Potassium ions are non-essential activators of several aldehyde dehydrogenases (ALDHs), whereas a few others require the cation for activity. Two kinds of cation-binding sites, which we named intra-subunit and inter-subunit, have been observed in crystal structures of ALDHs, and based on reported crystallographic data, we here propose the existence of a third kind located in the central cavity of some tetrameric ALDHs. Given the high structural similarity between these enzymes, cation-binding sites may be present in many other members of this superfamily. To explore the prevalence of these sites, we compared 37 known crystal structures from 13 different ALDH families and evaluated the possible existence of a cation on the basis of the number, distance and geometry of its potential interactions, as well as of B-factor values of modeled cations obtained in new refinements of some reported crystal structures. Also, by performing multiple alignments of 855 non-redundant amino acid sequences, we assessed the degree of conservation in their respective families of the amino acid residues putatively relevant for cation binding. Among the ALDH enzymes studied, and according to our analyses, potential intra-subunit cation-binding sites seem to be present in most members of ALDH2, ALDH1L ALDH4, ALDH5, ALDH7, ALDH10, and ALDH25 families, as well as in the bacterial and fungal members of the ALDH9 family and in a few ALDH1, ALDH6, ALDH11 and ALDH26 enzymes; potential inter-subunit sites in members of ALDH1L, ALDH3, ALDH4 from bacillales, ALDH5, ALDH7, ALDH9, ALDH10, ALDH11 and ALDH25 families; and potential central-cavity sites only in some bacterial and animal ALDH9s and in most members of the ALDH1L family. Because potassium is the most abundant intracellular cation, we propose that these are potassium-binding sites, but the specific structural and/or functional roles of the cation bound to these different sites remain to be investigated.