None of these contributions can be neglected in a dense multi-component system, such as the coacervate phase, therefore, their estimation remains the main theoretical challenge. For example, many theoretical models and simulations predict that monovalent ions should preferentially accumulate in the coacervate phase whereas experiments show that simple salts, such as NaCl, slightly prefer the supernatant phase. It took several decades to establish a qualitative explanation of this observation. While many theories and experiments investigated the partitioning of monovalent salts, much less is known about the partitioning of multivalent ions or small organic molecules. In brief, these experimental studies have shown that multivalent ions generally tend to accumulate in the polyelectrolyte complex phase. Furthermore, the partitioning of multivalent solutes depends also on their size,hydrophobicity and charge.