A variety of charge transfer effects can take place as a result of core ionization of anion-molecule complexes. As specific examples we study the core ionization of Na-H2O and Cl-H2O clusters using an ab initio Green's function method. The site-localized character of the electron distribution in the ground state of these weakly bound clusters favors a transfer of an electron from the negative atomic ion to the H2O molecular unit upon ionization of the O1s core level. The charge transferred screens the created core hole giving rise to the appearance of low-lying satellites which are completely absent in the spectrum of the isolated H2O molecule. Energies and intensities of the charge-transfer satellites are found to depend strongly on the chemical type of the atomic anion. While the bandshape of the O1s(-1) spectrum of Cl-H2O is very similar to that of isolated H2O, the spectrum of Na-H2O has virtually nothing to do with the core-ionization spectrum of the water molecule. Interestingly, the charge-transfer satellites seen in the O1s core-ionization spectrum of the Cl-H2O cluster can be well related to states in the O1s core-excitation spectrum of the isolated H2O molecule. The physics of core ionization of the clusters is discussed in detail. (C) 2003 American Institute of Physics.