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We report the first systematic assessment of intrinsic photothermal stability of a large panel of complex lead halides APbX₃ incorporating different univalent cations (A = CH₃NH₃⁺, [NH₂CHNH₂]⁺, Cs⁺) and halogen anions (X = Br, I) using a series of analytical techniques such as UV–vis and X-ray photoelectron spectroscopy, X-ray diffraction, EDX analysis, atomic force and scanning electron microscopy, ESR spectroscopy, and mass spectrometry. We show that heat stress and light soaking induce a severe degradation of perovskite films even in the absence of oxygen and moisture. The stability of complex lead halides increases in the order MAPbBr₃ < MAPbI₃ < FAPbI₃ < FAPbBr₃ < CsPbI₃ < CsPbBr₃, thus featuring all-inorganic perovskites as the most promising absorbers for stable perovskite solar cells. An important correlation was found between the stability of the complex lead halides and the volatility of …