Quantum yields for photochemical nitrogen extrusion from seven 1-pyrazolines were found to range from 0.12 to 0.88 but 4-oxo-3,3,5,5-tetramethyl-1-pyrazoline (8) proved to be photochemically and thermally remarkably stable. 4-Isopropylidene-3,3,5,5-tetramethyl-1-pyrazoline (7) decomposed thermally with Ea = 39.8 kcal mol−1 and ΔS≠ = 0.7 e.u. The high stability of this compound contrasts with 4-methylene-1-pyrazoline (17).