CO stretching potentials were obtained by ab initio multireference CI calculations for singlet valence and Rydberg states of H2CO in CzV symmetry. Within the Franck-Condon region, the valence states 'Bl(o,n*) and 'Al(n,n*) cross all members of the n -Ryd series as well as 'Bl(n,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 lA~(n,n*) becomes the ground state at large R(CO), n,n* is capable of coupling all 'AI Rydberg states with the ground state, which dissociates to CH2 + 0 fragments at 7.64 eV. Thus, the observed absorption continuum above 7.5 eV is attributable to predissociating interactions. In the vertical region, the 'Al(n,n*) valence state lies at 9.6 eV and exhibits a heavy mixing with the no,n*2 configuration. The reported "n,3d" band at 8.88 eV is reassigned to the 0-0 band of the 3'A1 -%'Al transition, where the minimum of 3'Al results from an avoided crossing between n,n* and n,3p,. This explains the apparent large quantum defect and other anomalies reported for the so-called "n,3d" band.

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