Atomic mercury subjected to mercury arc irradiation reacts in solid hydrogen to give the linear HgH2 molecule with strong IR absorptions at 1902 and 773 cm(-1). Annealing leads to HgH2 dimer and trimer, and warming above 7 K allows the hydrogen matrix to sublime and solid HgH2 to form. This covalent molecular solid is characterized by strong IR absorptions at 1802 and 673 cm(-1) and by decomposition at 150-170 K.