The aza-Diels-Alder reaction of 4-phosphinyl- and 4-phosphonyl-1,2-diaza-1,3-butadienes with styrene, cyclopentadiene, dihydrofuran and norbornadiene is reported. Monocyclic, bicyclic and polycyclic tetrahydropyridazines containing a phosphane oxide or a phosphonate substituent are obtained. The formation of cycloadducts can be explained by an endo transition state in the cycloaddition process of these heterodienes with styrene, cyclopentadiene, dihydrofuran, and by means of an exo transition state in the case of norbornadiene. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheirn, Germany, 2005).