Frozen-core and all-electron CCSD(T) calculations have been performed in order to derive accurate geometrical characteristics as well as dimerization energies of the two lowest-lying isomers of formaldehyde dimer (H2CO)(2): dimer I (C-s) and dimer II (C-2h). Contrary to early MP2 calculations, it has been unambiguously determined on the basis of CCSD(T) complete-basis-set extrapolations that dimer I is the true global- minimum structure, which lies 0.3-0.4 kcal/mol lower in energy than the dimer II structure at 0 K. The obtained equilibrium geometries and dimerization energies can serve as a benchmark for testing the performance of other, less computationally demanding methods. (C) 2013 Elsevier B.V. All rights reserved.