A novel, thermally curable, tri-component organic glass for electro-optic applications was designed and synthesized. The system employed the Diels–Alder cycloaddition reaction to effect efficient cross-linking. The first component was a dendrimer containing multiple electro-optic chromophore substituents surrounded by an outer periphery possessing diene functionality. The second was a furan-protected, bis-dienophile electro-optic chromophore, introduced in order to function as a nonlinear optically-active cross-linking agent. The initial glass-transition temperature of the material was tuned by the addition of the third component, an optically-inert, maleimide-based dienophile cross-linking agent. Tuning of this mixture allowed optimum poling temperature to coincide with optimum thermal conditions for promotion of the Diels–Alder cross-linking reaction. The electro-optic properties of the material were evaluated in real-time, using a reflection-based single-beam ellipsometry apparatus that was modified to perform in situ signal monitoring. The high electro-optic activity observed (r₃₃ of 150 pm V⁻¹), was thermally stable up to 130 °C (a 48 °C improvement over similar uncross-linked materials). After processing, materials were insoluble in acetone, retained 90% of their original r₃₃ after 15 months at room temperature, and performed well in accelerated operational testing at 85 °C in air.