Abstract: Cross-conjugated mesomeric betaines play an important role in heterocyclic synthesis. They can act as 1,4-dipoles in cycloaddition reactions or can be rearranged at higher temperatures to thermodynamically more stable compounds [2,3,4]. The rearrangement of cross-conjugated mesomeric pyrimidines has been extensively studied. When substituted on nitrogen with an aryl substitutent they can be rearranged to 2,3-disubstited 4-quinolones via unsaturated ?-lactame intermediates [2,3,5] (type A rearrangements [3])or via 2-oxoketenes to 4-hydroxy-2-quinolones (type B rearrangements [2]) [2,3,4,6]. In this paper we want to present type B rearrangements of pyridazino[2,3- a]pyrimidines in which the intermediate 2-oxoketene has two possibilities for ringclosure reactions to lead to thermodynamically more stable compounds. The thermolysis of pyrido[1,2-a]pyrimidines has been described before [6]. For this investigation two typs of pyridazino[2,3-a]pyrimidines were required. One with (the usual) electron-deficient pyridazine moiety (4) and one with with an electron rich pyridazine nucleus (7), i.e. with a high electron density at position 9 to allow an electrophilic attack of the expected ketene intermediate.