Polycarboxylic ether-based high-range water reducer (PCE) has been proposed to use due to the operational advantages of reduced water content and increased fluidity of cementitious mixtures. But the concern about using PCE can increase the mobility of radionuclides as well. Nuclear Decommissioning Authority (NDA) showed that the PCE formulations increased radionuclide solubility in free solution. Solubility of U(VI), 239Pu, 241Am with the cementitious materials tested with 3:1 pulverized fuel Ash/Ordinary Portland Cement (PFA:OPC) and 9:1 Ground Granulated Blast Furnace Slag/OPC (GGBS:OPC) with PCE that increased at least one and, in some cases, more than three orders of magnitude (between 10-9 and 10-4 mol dm-3) for these radionuclides in the cement-equilibrated solution. It is possible that the relatively low molecular weight substances present in the PCE cement mixture increase the solubility of radionuclides. In addition, the organic substances that are easily miscible with water can contribute to increase the solubility. In this study, several radionuclides (Nb, Ni, Pd, Zr, and Sn) that may be present in intermediate and low-level waste (LIW) repositories were selected based on the half-life and the estimated dose accordingly, and the solubility tests were conducted with and without PCE in solution. To simulate the field condition of the underground repository, synthetic groundwater was prepared based on the recipe by the KAERI Underground Research Tunnel (KURT) DB-3 GW and used as a solvent. The solubility limiting solid phase (SLSP) of each radionuclide was determined using Geochemist’s WorkBench (GWB) model. The selected solid phases are Ni(OH)2, ZrSiO4, Nb2O5, Pd(metal), and SnO2, respectively, and the solubility experiments were conducted with 1.0wt% of PCE per total weight and 0.5 g / 250 ml of selected radionuclide’s SLSP for 90 days at room temperature (25°C). Compared with and without PCE presence in solution, the selected radionuclides also showed an increased solubility with the presence of water reducing agent like PCE. This results can be used to correctly estimate the mobility of target radionuclides with the presence of PCE in repository environments.