Resonant IR excitation of gas-phase molecules and clusters can lead to superhot species that thermally emit an electron. Monitoring the mass selected ion yield as a function of IR laser frequency yields the IR-REMPI (infrared resonance enhanced multiphoton ionization) spectrum of the molecule or cluster. Although this IR-REMPI spectrum is not the same as the linear absorption spectrum, it can be quite similar and it yields valuable information on the IR optical properties of the species investigated. In this article, the method and the necessary tools are presented. Results from experiments on fullerenes, metal carbide, metal oxide, and metal nitride clusters are shown.