The equilibrium structures, the stabilization energies, the harmonic vibrational spectra, and the infrared intensities of linear and cyclic hydrogen cyanide, (HCN)(n), and cyanoacetylene oligomers. (HC3N)(n), were calculated at the ab initio self-consistent field and at the Moller-Plesset second-order level, as well as with the aid of a density functional method, Several extended basis sets were applied. The systematic modifications of the most important properties characteristic for the C-H---N hydrogen bond in these two series of intermolecular clusters, in particular, the intermolecular distances, R(H--N), the intramolecular distances, R(C-H), the interaction energies per hydrogen bond, with and without zero-point energy corrections, the C-H stretching frequencies, nu/(C-H), and their corresponding infrared intensities were monitored as a function of the oligomer size and are discussed in detail. The mode of convergence to the infinite chain limit is described and found to be qualitatively quite similar in both systems. From a quantitative point of view, all features usually attributed to hydrogen-bond nonadditivity are somewhat weaker or the cyanoacetylene clusters, mainly a consequence of the molecular size. Tentative assignments are suggested for the infrared active vibrations of larger cyclic cyanoacetylene clusters.