Methods of forming nonvolatile memory devices include forming first and second floating gate electrodes of first and second nonvolatile memory cells, respectively, at side-by-side locations on a substrate. The substrate is selectively etched to define a trench therein extending between the first and second floating gate electrodes. The trench is at least partially filled with a first electrical insulation pattern. An inorganic polysilazane-type spin-on-glass (SOG) layer is conformally deposited on the first and second floating gate electrodes and on the first electrical insulation pattern and then partially removed.