CO stretching potentials were obtained by ab initio multireference CI calculations for singlet valence and Rydberg states of H2CO in C-2v, symmetry. Within the Franck-Condon region, the valence states B-1(1)(sigma,pi*) and (1)A(1)(pi,pi*) cross all members of the n --> Ryd series as well as B-1(1)(pi,3s). The structures of the absorption bands are therefore extensively altered by vibronic mixing, intensity borrowing, and predissociative interactions. This explains why the n --> Ryd bands are observed to be vibrationally more complex and of higher intensity than expected. Since (1)A(1)(pi,pi*) becomes the ground state at large R(CO),pi,pi* is capable of coupling all (1)A(1) Rydberg states with the ground state, which dissociates to CH2 + O fragments at 7.64 eV. Thus, the observed absorption continuum above 7.5 eV is attributable to predissociating interactions. In the vertical region, the (1)A(1)(pi,pi*) valence state lies at 9.6 ev and exhibits a heavy mixing with the n(0),pi*(2) configuration. The reported "n,3d" band at 8.88 eV is reassigned to the O-O band of the 3(1)A(1) <-- ($) over tilde X(1)A(1) transition, where the minimum of 3(1)A(1) results from an avoided crossing between pi,pi* and n,3p(y). This explains the apparent large quantum defect and other anomalies reported for the so-called "n,3d" band.