Gas phase thermolysis of β-N,N-dimethylhdrazinopropenoates (1) provides a new route to pyrazolinones (2) involving a [1,4] methyl shiftfrom nitorgen to carbon in the pyrazolinium ylide intermediates (3); the mechanism is supported by the isolation of ylides (3a,b,e), the structures of (3a) and (3e) being confirmed by X-ray crystallography.