We have investigated by means of HREEL spectroscopy electron induced reactivity in a binary CO2 : NH3 ice mixture. It was shown that the interaction of low energy electrons (9-20 eV) with such mixtures induces the synthesis of neutral carbamic acid NH2COOH and that flashing the sample at 140 K induces the formation of ammonium carbamate. The products have been assigned by FTIR spectroscopy of a CO2 : NH3 mixture heated from 10 K to 240 K. A mechanism involving dissociation of NH3 molecules into NH2* and H* radicals is proposed to explain the product formation.