We have developed a new laboratory approach to investigate the evolution of interstellar ices by separating thermal processing from VUV processing. Infrared spectroscopy and mass spectrometry are used to monitor the thermal evolution of ice containing formaldehyde (H2CO) and ammonia (NH3). The main idea is to distinguish photon-induced chemistry from thermally induced chemistry. We show that H2CO and NH3 thermally react at low temperature (40 K) to give aminomethanol (NH2CH2OH). We also show that in presence of HCOOH the warming of an ice mixture H2CO:NH3 gives at room temperature hexamethylenetetramine (HMT-C6H12N4). From an astrobiological point of view, HMT is of significant interest, since its hydrolysis leads to the formation of amino-acids. Furthermore, it is one of the main refractory products obtained after VUV photolysis at 10 K of ice mixtures containing CH3OH and NH3. It is also suspected to be a part of the organic matter present in comets and asteroids. In this contribution, we demonstrate that HMT can no longer be considered as a reliable indicator of VUV photolysis of ice, as we demonstrate that it can be formed from a purely thermal reaction.