The geometries, relative stabilities and proton affinities for the different tautomers of 2-, 3- and 4-hydroxyquinoline derivatives and their thio and azo analogs along with their fixed forms (i.e. model molecules in which the proton migration is eliminated) were calculated with full geometry optimization using AM1, PM3 and MNDO methods. The predominance of oxo forms over hydroxy forms were confirmed with all three methods both in gas and liquid phases, as cited in the literature, with the exception of 3-hydroxyquinoline for which the AMI and MNDO methods both in gas and liquid phases suggest the predominance of the hydroxy form. For the thio analogs the predominance of thione forms over mercapto forms and for the amino analogs the predominance of the amino forms over imino forms were confirmed both in gas and liquid phases with all three methods as indicated in the literature. (C) 2002 Published by Elsevier Science B.V.