The infra‐red absorption spectra of the purified rubber hydrocarbon fractions of several naturally occurring elastomers (hevea, guayule, kok‐saghyz, gutta‐percha, balata, and chicle) have been determined at room temperature over the range 2–15&mgr; and it has been shown that the elastomers can be classified on the basis of the spectral data as belonging to one or the other of two different types, namely the hevea type or the gutta type. Both types of elastomers have also been studied over the temperature range −25 to +80°C and reference spectra for both the amorphous and the solid (crystalline) phase(s) occurring in this range have been determined for each type. At room temperature the spectral differences between the hevea (amorphous state) and gutta (&agr;‐ or &bgr;‐crystalline modifications) types are much more pronounced than is the case when the spectra of both types in the same (amorphous) states are compared.The spectra of two pair ofcis‐trans‐isomers containing the structural unit of polyisoprene have been determined and correlated with the elastomer spectra. The results indicate but do not prove that hevea‐ and gutta‐type elastomers have the molecular structure ofcis‐ andtrans‐polyisoprene, respectively.The infra‐red absorption spectra of oriented crystalline films of hevea rubber and of gutta‐percha in both the &agr;‐ and &bgr;‐forms have also been obtained over the range 2–15&mgr; using polarized radiation. The observed dichroic effects lead directly to the conclusion that hevea‐ and gutta‐type elastomers arecis‐ andtrans‐polyisoprene, respectively. Although it has not been possible to deduce the exact spacial configuration of the molecules in the crystallites, it has been indicated that the planar isoprene models are unsatisfactory and that the results are in close agreement with the twisted models proposed on the basis of x‐ray diffraction.Further use has been made of the polarization data in proposing nearly complete but tentative interpretations of the spectra of both hevea‐ and gutta‐type elastomers. The possibility of making use of the vibrational assignments for the interpretation of spectral changes produced by oxidation or other forms of degradation is pointed out.