The stability of the 124 superconductive phase YBa2Cu4O8 upon exposure to air and saturated humidity at ambient temperature has been studied by thermogravimetry, X-ray diffraction, and ac susceptometry. Extent of phase conversion was monitored by TG and confirmed by XRD and ac susceptometry. 124 samples upon prolonged exposure to air were found to be no longer phase-pure, with partial conversion to 123 and CuO. On oxygen annealing, reconversion of 123 + GuO to 124 was observed. However, upon prolonged exposure to saturated humid conditions, phase-pure 124 dissociated irreversibly into 211, GuO, and a highly disordered 124-like structure with planar defects along many hkl indices and was found to be nonsuperconducting even up to 60 K.