MP2 and CCSD(T) ab initio calculations have been carried out to elucidate geometrical structure and vibrational frequencies of representative lanthanide trihalides LnX(3) (Ln = La, Lu; X = F, Cl) explicitly including temperature, anharmonic, inert-gas matrix, and spin-orbit effects. The results have been compared with gas-phase electron diffraction, gas-phase IR measurements, and IR spectra of molecules trapped in inert-gas matrices. On the Born-Oppenheimer surface LaCl3, LuF3, and LuCl3 adopt trigonal planar (D-3h) geometry while LaF3 assumes a slightly pyramidal (C-3v) structure. Because of normal-mode anharmonicities, the resulting thermal average bond angles are considerably lower than the equilibrium ones, while vibrationally averaged bond lengths are predicted to be longer. The inert-gas matrix effects, modeled by the coordination of two inert-gas molecules LnX(3)center dot IG(2) (IG = Ne, Ar, Xe, and N-2) are substantial and strongly depend on the polarizability of coordinating particles. Coordinating inert-gas units always favor the tendency of LnX(3) molecules to adopt planar structure and induce noticeable frequency shifts.