Triplet and quintet states of formaldehyde, H2CO, have been investigated using multireference CI methods and triple-zeta + polarization + diffuse basis sets. For triplet states, vertical excitation energies, C-O stretch potentials in C-2 upsilon symmetry, and out-of-plane potentials (C-s) are given. Geometry optimizations confirmed the nonplanarity of (3)(n,pi*), (3)(sigma,pi*), and (3)(pi,pi*). For (3)(n,pi*), the C-O distance and out-of-plane angle are in good agreement with experimental values, but for (3)(pi,pi*) both the calculated distance and frequency suggest larger values than reported experimentally. States of Rydberg character are planar and are expected to be predissociated by 1(3)A(l)(pi,pi*) and 1(3)B(l)(sigma,pi*). Franck-Condon (FC) factors for absorption from the ground state into (3)(sigma,pi*) and (3)(pi,pi*) were calculated, with the latter showing good agreement with observed energy-loss spectra. Furthermore, oscillator strengths and radiative lifetimes were evaluated for triplet-triplet systems originating from (3)(n,pi*), (3)(pi,pi*), and (3)(sigma,pi*). Dipole moments are given for the three triplet valence states (polarity H2C+O-) as well as for (3)(n,3s) (H2C-O+). Various predissociation and vibronic coupling mechanisms are discussed. C-O stretch potentials in C-2 upsilon symmetry show that quintet states with n pi,pi*Ryd configurations are stable, forming a Rydberg series that converges to (IB2)-B-4(n pi,pi*) of H2CO+. Excitations into sigma*pi*, however, lead to repulsive potentials that cross the Rydberg series. All quintet minima lie well above their dissociation channels.