Results for the spectroscopic constants of the monohydrides, monoxides, and monofluorides of the lanthanide elements lanthanum and lutetium as well as the actinide elements actinium and lawrencium from all-electron and valence-only ab initio electronic structure calculations are presented. The valence-only investigations were carried out using energy-consistent pseudopotentials for the heavy atoms, Electron correlation was accounted for by means of the coupled-cluster method, Multireference averaged coupled-pair functional calculations were carried out to describe effects due to the mixing of the energetic close-lying ns(2-m)(n-1)d(m) (0 less than or equal to m less than or equal to 2) configurations of the lanthanide (n = 6) and actinide (n = 7) cations, The counterpoise correction was applied to estimate the size of the basis set superposition errors, Nonrelativistic as well as quasirelativistic calculations were performed, The latter included also corrections for spin-orbit effects for the actinide compounds derived from limited configuration interaction calculations. The reliability of the pseudopotential approach is demonstrated by a comparison to the results of corresponding nonrelativistic and scalar-relativistic Douglas-Kroll-Hess all-electron self-consistent-field calculations performed with large basis sets, The influence of shell-structure effects, i.e. the filling of the If and 5f shell, and relativistic effects on the molecular properties is discussed, The values for the lanthanide and actinide contractions are found to depend strongly on the ligand and to vary from 6 to ii pm for lanthanides and from 11 to 17 pm for actinides, Relativistic effects play a significant role; for example, their neglect even leads to a slight lanthanide/actinide expansion in the case of the monoxides.