State-selective (VUV + UV) resonant multiphoton ionization is used to measure the state- and energy-distributions of the gas-phase products which accompany the UV photoinduced polymerization of formaldehyde on Ag(lll). At 355 nm, only direct desorption of H2CO is observed, whereas both CO product formation and H2CO desorption are found at 266 nm. The rotational state and translational energies of the desorbed CO products exhibit a "fast" channel associated with a prompt fragmentation process and a "slow" channel in near-thermal equilibrium with the Ag surface. No gas-phase hydrogen products (H atoms, H-2) were detected during UV exposure, but H-2 desorption is observed during breakup of the polymer above 200 K. The time-dependent desorption yields for both slow CO and H2CO are strongly temperature dependent and reflect the rate of polymerization. A simple kinetic model is used to describe the competing processes of desorption, fragmentation, and polymerization which ultimately result from the decay or fragmentation of an H2CO(a)(-) Or other H2CO(a) excited-state species. In combination with depletion rates measured over the temperature range 30-95 K, the kinetic model results in an activation energy for polymerization of 0.90 +/- 0.06 kcal/mol.