Spin-orbit effects on the spectroscopic constants (bond lengths, dissociation energies and harmonic vibrational frequencies) for HX, X-2 (X = Cl, Br and I) and IZ (Z = F, Cl and Br) molecules have been studied using shape-consistent relativistic effective core potentials (RECPs) with effective one-electron spin-orbit operator at HF, MP2, CCSD and CCSD(T) levels. Basis sets of pVTZ quality have been derived for Cl, Br and I for the present work. The spectroscopic constants calculated by the two-component RECP method are in good agreement with those from all-electron Dirac-Coulomb calculations with the basis sets of the similar quality at all levels of theory considered, suggesting that RECP methods mimic all-electron Dirac-Coulomb methods calculations rather well for molecules. Spin-orbit effects elongate the bond lengths, while reduce the dissociation energies and harmonic vibrational frequencies. From the spin-orbit effects on the spectroscopic constants, especially on dissociation energies of IF, ICl, IBr and I-2, it is confirmed that the magnitude of spin-orbit effects increases in the F < Cl < Br < I order. Spin-orbit effects constitute a significant portion of relativistic effects for the studied molecules. For the dissociation energies of the studied molecules, the spin-orbit effects and electron correlation effects are slightly non-additive, implying the need of spin-orbit calculations at the correlated level for the high accuracy. (C) 2004 Elsevier B.V. All rights reserved.