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Abstract This thesis deals with the rotation-vibration theory and high resolution infrared spectroscopy of semirigid C3 molecules. Semirigid molecules form a class of molecules which are strongly bound with one well defined structure, and without low frequency internal motions. The theory, as well as the experimental studies of semirigid molecules are of special importance in the field of rotation-vibration spectroscopy. They provide a good starting point for interpreting and analyzing the spectra of practically all types of molecules. In this work, the theory is reviewed fromthe standpoint of one particular molecule, 13CH3I, which is a prolate symmetric top with C3 symmetry. The origin and the properties of the rotation-vibration Hamiltonian are discussed in detail. Molecular symmetry plays an important role in these studies. The expansion of the Hamiltonian for nuclear motion in powers of the vibrational operators converges rapidly as numerical examples thoughout the treatment indicate. The molecule is thus a good subject for the perturbation calculations, also reviewed here in detail. 13CH3I can be considered as a model example of semirigid molecules. From the spectroscopic point of view, this thesis is a study of the six fundamental bands of 13CH3I. The rotational analysis of the vibrational ground state is first given. Special attention is paid to obtaining the axial rotational constants which are problematic for symmetric top molecules. The relatively high energy level density of 13CH3I leads to several resonances. The fundamental bands, especially the higher ones, must therefore be treated as parts of band systems. Care is paid to properly take into account the effects of the near-lying vibrational levels on the constants of the fundamentals. Certain ambiguities in the rotation-vibration Hamiltonian of 13CH3I are also discussed.