The solubilities of naphthalene in ammonia could be measured at various temperatures and pressures above the critical point of ammonia to represent the relationship between the solubility and the density of ammonia at desired temperature and pressure by means of a simple equation. This equation allows a calculation of the solubilities at higher and lower pressures. Using the equation, the solution energies, the solution entropies, and the second cross virial coefficients between naphthalene and ammonia have been determined to be compared with those in the case of the dissolution of naphthalene in supercritical carbon dioxide.