Spin-orbit effects on the bond lengths and dissociation energies of sixth- and seventh-row p-block element monohydrides MH(M = Tl-Rn and element 113-118) are evaluated using relativistic effective core potentials at the coupled-cluster level of theory. Spin-orbit effects play a dominant role in the determination of molecular properties for the seventh-row hydrides. Spin-orbit effects on the bond lengths and dissociation energies of seventh-row hydrides are qualitatively similar to, but substantially larger than those of the sixth-row homologs due to the enormous spin-orbit splitting of 7p orbitals. Spin-orbit interactions change the bond lengths of sixth- and seventh-row hydrides by -0.02 similar to +0.03 Angstrom and -0.21 similar to +0.21 Angstrom, respectively. Spin-orbit interactions usually elongate the bond lengths except for the molecules of the (p(1/2))(1)-valence atoms, i.e., TlH and (113)H. The maximum elongation is predicted for (115)H, where the element 115(eka-bismuth) has the (7p(3/2))(1) configuration outside the inner (7p(1/2))(2) closed-shell. The spin-orbit coupling weakens the bondings between the heavy element and the hydrogen except for BiH and changes the dissociation energies by -0.71 similar to +0.08 eV and -2.18 similar to -0.23 eV for sixth- and seventh-row hydrides, respectively. The dissociation energy of the (114)H molecule is merely 0.39 eV, because the element 114(eka-lead) has a closed-shell electronic structure in the jj-coupling scheme. The bonding between the element 118(eka-radon), which is another closed-shell atom, and hydrogen is very weak and can be regarded as a pure van der Waals bond. But with highly electronegative elements the element 118 seems to form more stable compounds than other closed-shell atoms such as the element 112(eka-mercury) or the element 114. (C) 2000 American Institute of Physics. [S0021-9606(00)30606-7].