The infrared optical constants, n and k, (n˜ = n − ik) of swelling and nonswelling coals, their respective char products, and limestone at room temperature have been obtained by a particle extinction technique using compressed KBr tablets. By assuming spherical particles and using Mie theory for the particle extinction measurements in conjunction with normal specular reflectance measurements from polished specimens, it is possible to obtain more accurate values for n and k than if the usual Fresnel reflectance technique is misapplied to samples that may appear specular but do not satisfy the stringent conditions necessary for invoking the Fresnel equations. Values of k for coal were found to be an order-of-magnitude smaller than previously reported values, with absorption due mainly to molecular vibration. The results for char evidenced that a significant increase in absorption by free electrons takes place upon devolatization and carbonization of coal. The limestone results demonstrated pronounced molecular absorption bands characteristic of CaCO3.