The photochemistry of formaldehyde (H2CO) adsorbed on CO-saturated Cu(100) at 85 K was studied using electron energy loss spectroscopy (EELS) and temperature-programmed desorption (TPD). Formaldehyde was weakly adsorbed on CO/Cu(100) desorbing at 104 K with a desorption energy of 18.2 (+/-0.8) kJ/mol. Irradiation of the H2CO/CO/Cu(100) surface caused the molecularly adsorbed H2CO to polymerize, forming poly(oxymethylene) (POM). Irradiation also caused the formation of ethylene glycol (CH2OH)(2). Losses observed at 870 and 3365 cm(-1). after UV irradiation, were assigned to v(CC) and v(OH) modes, respectively, of (CH2OH)(2), indicating ethylene glycol was formed promptly upon irradiation. The presence of (CH2OH)(2) was confirmed by studying the adsorption of (CH2OH)(2) on clean and oxygen-covered Cu(100). The formation of ethylene glycol was likely governed by geometric constraints present within the formaldehyde overlayer.