On Ag(111) at 120 K, there is no thermal dissociation of methanal, but electron irradiation induces fragmentation. After irradiation of monolayers, temperature-programmed desorption (TPD) contains water (at 172 K), methane (190 K), hydrogen (200 K), and formaldehyde (225 and 255 K). X-ray photoelectron spectroscopy (XPS) intensities before and after irradiation are comparable, indicating little product ejection during irradiation. For multilayers there is an additional product in TPD-glycolaldehyde or methyl formate. During irradiation, the work function rises, reflecting adsorbate structural changes. The XPS, TPD, and work function measurements return to the clean surface values upon heating from 120 to 450 K. Evidently, impact ionization initiates the chemistry, and the resulting cations fragment, reneutralize, and chemisorb to Ag. Dissociation of a C-H bond to form H2C(a)O-(a)H or a C-O bond to form C-(a)H-3 and O-(a)H accounts for the products found after relatively short irradiations. Evidence for O-(a)H bond breaking by electron irradiation appears only after long irradiation times, Formaldehyde oligomers, stabilized by interactions with Ag, are proposed to account for the H2CO desorption at 255 K.