The group of three lowest singlet C 1s-excited states of formaldehyde H2CO is studied theoretically. The equilibrium geometries are determined at the restricted open-shell Hartree-Fock (ROHF) level and refined total energies are obtained using the multireference configuration interaction (MRCI) approach. In agreement with an earlier prediction [Chem. Phys. 122, 9 (1988)] the second lowest singlet state, B-1(2), is characterized by a doubly excited, "two particle-two hole" (2p-2h), configuration C 1s,n --> pi (*2). Our calculations predict that H2CO in the B-1(2)(2p-2h) state has a stable pyramidal equilibrium structure with a barrier to inversion of 0.28 eV, the valence angle being close to 107 degrees. The calculated length of the CO bond is 1.390 Angstrom. The B-1(2)(2p-2h) state is shown to be also bound with respect to all possible dissociation and rearrangement processes. The lowest predicted dissociation energy for the B-1(2) state (H2CO*-->H-2+CO* reaction) is 0.29 eV (6.69 kcal/mol). The rationalization of the great stability of the B-1(2)(2p-2h) state is the similarity of its electronic structure to that of the first singly-excited state (2)A(')(n) of nitroxyl radical H2NO .. The neighboring states B-1(1)(C 1s --> pi*) and (1)A(1)(C 1s --> 3s) are characterized within the same framework. Spectroscopic implications and possibilities for the experimental identification of the B-1(2)(2p-2h) state are discussed.