The electronic structures of the tetramethyldihydropyrazoles [(I; R=Se,S:O, NNH2, NN:CMe2) and II] have been studied by photoelectron spectroscopy and interpreted with the aid of semi-empirical SCF MO calcns. (PM3). Gas-phase pyrolyzes of I and II have been carried out and analyzed by photoelectron spectroscopy. Both I(R =Se, S:O) and in part I(R=NNH2) eliminate mol. nitrogen to form reactive species which cyclize to three-membered rings, i.e. the novel alkylideneselenirane III, the alkylidenethiirane S-oxide IV, and the cyclopropanone hydrazone V, resp., which decomp. at higher temps. The elusive alkylideneselenirane III was studied by IR spectroscopy in an argon matrix at 10 K.