The structures and vibrations of monolayers of both normal and para-H-2 as well as of HD and D-2 on single-crystal NaCl(001) surfaces have been studied with high-resolution helium atom scattering at surface temperatures of 8 K. The diffraction patterns of all of these systems are consistent with a (1 x 1) structure, thereby simplifying the interpretation of the time-of-flight spectra. For p-H-2, a well-resolved dispersionless inelastic peak with DeltaE = 7.0 meV can be assigned to vibrations parallel to the surface and is accompanied by slightly anharmonic overtones at DeltaE = 11.8 and 17.2 meV. The fundamental adsorbate mode hybridizes at larger wave vectors with the Rayleigh mode of the substrate producing an additional branch at 8.5 meV. For ortho-H-2, the vibrational frequencies are greater by about 1 meV but are otherwise similar. The frequencies for HD and D-2 are largely consistent with the expected mass-induced shifts in the frequencies. There is also some evidence for rotational excitation of the adsorbed HD molecule.