Umemoto et al. have recentIy published a mechanism for the Hg(3Pt) photosensitisation of NHa, proposing that the short yavelength component of the humhrescence arises directly from a reaction of Hg$Pr) with NHa: H,~(~pr) + NH3 + HgNH r , conclu a HgNH$ + Hg f NH3 + hv(sbort A). From a detailed analysis of the kinetics, Calkar and Freeman bad previousIy ed that the carrier is an excited complex formed in a bimokcular reaction of HgcPo) and NHa. To reveal the identity of the essentiakprecursor, we have monitored the intensity of the short waveIength spectrum in experiments in which the relative concentrations of HgcPt) and Hg(3i?& were widely varied. AU the results are in excellent accord with the mechanism of Cal&ear and Freeman. Particufarly striking is the effect of adding N-J to small pressures of NH3 , which can increase the intensity by two orders of magnitude without significantly &ranging the Hg(3P1) concentration. From time resolved measurements we also conclude that any emission arising directly from reactions of Hg(3PI) is negiigrble. In the courseofthis research we have discovered that CO efficiently deactivates the stabiiised HgNHa (A& The rate coefficient is 2.040.1) X IO-lo cm3 molecuIe-f s-l_