This thesis describes the preparation of microcrystalline dispersions of a series of organic semiconductors (the alkali metal salts of tetracyanoquinodimethane) within a host polymer. The linear optical properties of these composites are found to be dependent on the method of preparation. Their third order nonlinear optical properties are then investigated by measurement of the intensity dependence of their transmision. Close to the low energy, local excitation band saturable absorption is observed. In the region of the charge transfer band reverse saturable absorption is observed. The results are discussed in terms of a two level model.