C:H films in the thickness range 1 to about 20 monolayers have been prepared by hydrocarbon-ion-beam deposition at Pt(100) and Pt(111) single-crystal surfaces covered with a graphite monolayer. The films were investigated by Auger electron, electron-energy-loss (EELS), and high-resolution electron-energy-loss spectroscopies to investigate their electronic and vibronic characteristics. The hydrogen content of the films was determined at [H]/[C]=0.05--0.5, depending on deposition parameters such as feed gas and target temperature. AES and EELS identify the films deposited at 350 K as being of a distorted graphitic nature with a carbon [${\mathit{sp}}^{2}$]/[${\mathit{sp}}^{3}$] ratio of about unity. Vibrational spectroscopy reveals that at the film surfaces H atoms are bound to C atoms in sp, aromatic ${\mathit{sp}}^{2}$, and ${\mathit{sp}}^{3}$ hybridization states. From the vibrational spectra, the presence of -C\ensuremath{\equiv}CH, (aromatic)=CH, and -${\mathrm{CH}}_{\mathit{x}}$, x=1,2,3 groups at the surface is inferred. Upon annealing, the films at higher temperatures, 500--1400 K, sp, ${\mathit{sp}}^{3}$, and ${\mathit{sp}}^{2}$ groups get destroyed sequentially, paralleled by evolution of hydrogen (major product) and hydrocarbon species (minor product) from the films. After annealing at 1040 K, vibrational spectra exhibit only a ${\mathit{sp}}^{2}$ CH-related C-H stretch band and, accordingly, from EELS increased ${\mathit{sp}}^{2}$ bonding in the annealed films is obtained. This stability sequence of ${\mathrm{CH}}_{\mathit{x}}$ groups at the carbon network is in accordance with the expectations drawn from kinetic and thermodynamic data of hydrocarbons. The chemical structure of the deposited C:H films is insensitive to the nature of the deposition ion, e.g., methane, ethane, ethylene, or benzene, from which it is suggested that specific C-H bonding in the deposition ions does not act as a precursor for the film structure. Films deposited in the submonolayer range exhibit primarily ${\mathit{sp}}^{3}$ C-H bonding in accordance with a simple picture of the initial growth of the films.