Degradation ofHgBa2CaCu2Oxsuperconducting thin films witha- andc-axis oriented normal to the substrates in water has been studied with electrical measurement, x-ray diffractometry, scanning electron microscopy, and energy dispersive x-ray spectrometry. We observed the different degradation behavior ofa- andc-axis orientedHgBa2CaCu2Oxthin films normal to substrates in water immersion experiments. Thea-axis oriented films are much more stable than thec-axis oriented films in water. After a 60 min immersion, the zero-resistance temperature of thea-axis oriented film is still above 110 K, but the zero resistance is not observed in thec-axis oriented film in water even for 5 min. The intensity of(00l)peaks is suppressed at one order of magnitude faster than that of(l00)peaks as the total immersion time increase. ©1997 American Institute of Physics.