Local vibrational mode (LVM) spectroscopy has been used to investigate the evolution of complexes related to interstitial carbon (Ci) atoms in low-temperature-irradiated silicon crystals containing different carbon and oxygen isotopes. It is found that in oxygen-rich Si crystals in the course of the formation of the stable interstitial carbon-interstitial oxygen defect (Ci Oi), three metastable complexes incorporating Ci and Oi atoms occur. Local vibrational modes for all these defects with different combinations of carbon and oxygen isotopes are determined. Density functional modeling studies have been used to investigate the structural, electronic, and vibrational properties of several Ci Oi complexes. In addition to the most stable Ci Oi center, other stable structures incorporating Ci and Oi atoms have been found which account well for the LVM spectroscopy and deep-level transient spectroscopy data. © 2008 The American Physical Society.