Diazoolefins tend to be highly reactive compounds that rapidly lose dinitrogen. So far, most experimental evidence for diazoolefins is indirect, via trapping experiments. Here we show that diazoolefins are observed to form in reactions of N-heterocyclic olefins with nitrous oxide. The products benefit from resonance stabilization, which enables isolation on a preparative scale, and comprehensive characterization, which includes crystallographic analyses. N-heterocyclic diazoolefins show a strong ylidic character, with a high charge density at the carbon atom next to the diazo group. Despite the presence of terminal N2 groups, N-heterocyclic diazoolefins display a good thermal stability, which surpasses that observed for most diazoalkanes. N-heterocyclic diazoolefins can bind transition and main group metal complexes without the liberation of dinitrogen, and spectroscopic data show that they are strong electron donors. They can also undergo reactions that involve the N2 group, as evidenced by cycloaddition reactions.