Magnesiumphthalocyanine (MgPc) is a blue pigment whose X-phase (MgPc/(H₂O)₂) is known to exhibit an intense near-IR absorption. Because of this, MgPc has attracted attention as a material useful for laser printers as well as optical disks. In the X-phase of MgPc just like in Y-form titanylphthalocyanine, the presence of water molecules in the crystal lattice plays an important role for the appearance of the near-IR absorption as well as photoconduction enhancement. Therefore, the effect of water desorption on the dark conductivity and photoconductivity at elevated temperatures has been investigated in evaporated thin films of the X-phase of MgPc. Both dark conductivity and photoconductivity are found to greatly diminish with the onset of water desorption around 150 °C. This is due to the scattering of charge carriers at defects formed by collapse of the vacancies of water molecules. Especially, the photoconductivity is quite sensitive to subtle structural changes due to thermal motion of water molecules in the lattice, even before the water desorption. It is therefore crucial to keep the water molecules firmly coordinated to the central metal for the stable operation of MgPc-photoreceptors.