The one-pot reaction of sterically hindered hydrazones with tellurium dichloride or tellurium tetrahalide in the presence of triethylamine in benzene afforded Δ3-1,3,4-telluradiazolines 1, a novel heterocycle, via 1,3-dipolar cycloaddition of telluroketones and diazo compounds, both generated in situ. The molecular structure of 1a was established by X-ray crystallographic analysis. The photolysis of