Solid-state masers, hitherto only functional at cryogenic temperatures within a magnetic field, have recently been shown to operate at room temperature and in the earth’s field. One of the key components in the room-temperature solid-state maser that has been developed is a gain medium where the stimulated emission occurs. Unlike the ruby gain medium in conventional masers, the new gain medium is an organic semiconductor co-crystal produced from pentacene doped p-terphenyl. Its excited triplet sublevels are degenerated without applying any magnetic field. Moreover, the population inversion of the uppermost sublevel is very high, leading to higher photon emission at very low doping concentrations. This thesis describes the optimization of the pentacene doping concentration to provide a maximum gain output while maintaining a long relaxation time. Single crystals of pentacene doped p-terphenyl were grown via Bridgman-Stockbarger method at various concentrations. Physical and electrical properties were characterized and the solubility limit of pentacene in p-terphenyl was found to be 2.0 mol\%, while the optimum doping concentration, in terms of crystal perfection and response was at 0.1 mol\%. However, growing organic semiconductors presents certain difficulties – it is a costly and time-consuming process, there is a risk of degrading crystals by heat and oxidisation and the larger crystals produced are imperfect. Here we use 1,3,5-tri (naphthyl) benzene, a glassy, transparent material as a host for the preparation of a gain medium and compare it with a p-terphenyl crystal host. It is found to dissolve pentacene easily and forms a clear glassy solid on rapid cooling. Using a glassy host avoids the lengthy crystal growth process. Using home-built electron paramagnetic resonance apparatus (EPR), a red-shift frequency to 1.426 GHz was observed. This compares with 1.45 GHz from the pentacene doped p-terphenyl. The transition bandwidth due to the inhomogeneous relaxation of pentacene doped 1,3,5-tri (naphthyl) benzene is broader than that of the pentacene doped p-terphenyl and extends to the hydrogen line. EPR results revealed that it has potential to be a gain medium for room-temperature solid-state maser. The development of gain media working at room temperature could pave the way for maser applications in fields such as security, communications as well as deep space communication.