Submonolayer formaldehyde (H2CO) has been photodissociated on Ag(111) at 35 K using nanosecond 287 nm laser pulses. The low temperature prohibits further bimolecular reactions of the dissociation products and allows their analysis by electron energy loss spectroscopy. One of the dissociation products has been identified as the radical species methylene (CH2). The dissociation is induced by photoexcited substrate electrons attached to formaldehyde, forming a transient formaldehyde negative ion that dissociates. Due to the influence of adsorbate-substrate bonding on reaction energetics, the dissociation channel of H2CO- on Ag appears to be different from that of the gaseous negative ion.