Attachment cross-sections of water molecules onto size selected protonated (H2O)nH+ and deprotonated (H2O)n − 1OH− water clusters have been measured in the size range n = 30-140 for 10 eV kinetic energy of the clusters in the laboratory frame. Within our experimental accuracy, the attachment cross-sections are found to have the same magnitude and size dependence for both species. It is shown that electrostatic interactions are likely to play a role even for the largest sizes investigated.