Stabilization/solidification (S/S) is one of the most employed techniques for treating liquid wastes containing heavy metals. Magnesium potassium phosphate cements have been used in recent years as stabilizing agents with positive results. However, little information is available about the retention of the metals by magnesium potassium phosphate cements matrices with the exception of the results of compliance tests. In this work, several pastes were prepared by reaction between low grade MgO and KH2PO4 in the presence of different heavy metal nitrate solutions (containing Cd(II), Cr(III), Cu(II), Ni(II), Pb(II) or Zn(II)). In all cases, the initial metal content of the dissolution was 25 g dm−3 and the oxide–phosphate ratio of the pastes was (50:50) in weight. Four different leaching tests were conducted on magnesium potassium phosphate cement pastes: simple batch test (EN 12457-2), equilibrium leaching test, availability test (NEN 7371) and acid neutralization capacity test (ANC). The metal leachate concentration was determined by means of ICP-MS. The stabilization of metals was successful in all cases, although the immobilizing system shows a better behaviour for Pb(II) and Cr(III) under acidic or neutral conditions.