Glass formation was observed in a wide region in the system Cu 2 O P 2 O 5 MoO 3 , except for compositions near the area of the binary Cu 2 O MoO 3 composition. The transformation temperature was mainly controlled by the packing density of the glass network. The structure of the glasses, which depends strongly on composition, was studied by infrared spectroscopy. The presence of various phosphate and molybdate structural units, from discrete monomeric to those with three-dimensional networks, was observed. P 2 O 5 - and Cu 2 O-rich glasses are composed mainly of (PO 3 - ) n chains, Cu 2 O- and MoO 3 -rich glasses are composed mainly of discrete MoO 4 2 - , Mo 2 O 7 2 - and PO 4 3 - anions and, finally, the structure of P 2 O 5 - and MoO 3 -rich glasses is predominantly built up by a three-dimensional network of the MoO 2 (PO 3 ) 2 type.