A process of converting 2,3,4,5-tetrahydropyrimidines (.DELTA.-1) having at least one hydrogen in the 2-position to 3,4,5,6-tetrahydropyrimidines (.DELTA.-2), which comprises hydrogenating and dehydrogenating said .DELTA.-1 tetrahydropyrimidines. Such hydrogenating and dehydrogenating may be done in either two sequential steps or in one step. Said .DELTA.-1 tetrahydropyrimidines can also be converted to .DELTA.-2 tetrahydropyrimidines by heating in the essential absence of a solvent, and preferably under basic conditions. In addition, corresponding hexahydropyrimidines can be converted to corresponding .DELTA.-2 tetrahydropyrimidines by dehydrogenation. This invention also relates to the use of .DELTA.-2 tetrahydropyrimidines, particularly as corrosion inhibitors.