Although both neurons and oligodendrocytes have been well studied individually, very little is known about how they interact with each other. New methods are needed to further study the intricacies of this interplay in terms of cellular and molecular dynamics. Spatial Light Interference Microscopy (SLIM) is a quantitative phase imaging technique that generates phase maps related to the dry mass content of the sample. In this work, we study the ability of SLIM to quantify myelination at the axonal level. We imaged a series of cocultures comprising hippocampal neurons and oligodendrocytes, of varying densities, using SLIM, and evaluated dry mass formation and growth of myelin.