An extension of the Flory-Huggins lattice theory to include charged polymers is described and utilized to predict the polymer solubility in solvent/polymer and solvent/polymer/salt systems and the polymer miscibility in solvent/polymer 1/polymer 2 systems. In the extended theory,all simple ions (counterions to the polymer and ions from the salt) are treated on the same level as the other components and the effect of the electrostatic interaction is included by imposing electroneutral phases. It was found that charging the polymer generally increases the polymer solvency or polymer miscibility. In those cases where both polymers carry charges of the same sign, the minimal miscibility is obtained at such conditions that the coexisting phases have similar counterion concentration. Generally, the increased solubility/miscibility could be attributed to the contribution from the mixing entropy of the small ions. Good qualitative agreement with several experimental investigations are found. The increased solubility of alkoxy polymers in aqueous solution upon an introduction of charged groups is also investigated.