A simple ab initio procedure is used to calculate the vibrational structure observed in the carbon 1 s ionization of seven hydrocarbons (methane, deuteromethane, ethane, ethene, deuteroethene, ethyne, and deuteroethyne), with good agreement between experiment and theory. The method involves use of the equivalent-cores approximation, localized holes in molecules with equivalent carbons, and the harmonic oscillator approximation. The approach provides insight into the vibrational modes of the core-ionized molecules. It is potentially useful in extracting carbon 1 s ionization energies from spectra from molecules having inequivalent carbons or in finding information on inner-hole lifetimes from inner-shell spectra.