The density of sputtered Ge2Sb2.04Te4.74 thin films upon annealing has been precisely determined by x-ray reflection and compared to the values determined from x-ray diffraction (XRD) data. The film density increases in two steps around 130 and 280 degreesC upon annealing up to 400 degreesC. These increases are consequences of phase transitions from amorphous to NaCl type and from NaCl type to hexagonal structure, respectively, as revealed by XRD. Average density values of 5.87+/-0.02, 6.27+/-0.02, and 6.39+/-0.02 g/cm(3) were measured for the amorphous, NaCl-type, and hexagonal phases, respectively. This corresponds to density changes upon crystallization of 6.8+/-0.2% and 8.8+/-0.2% for NaCl-type and hexagonal phases, respectively. The accompanying film thickness reductions were determined to be 6.5+/-0.2% and 8.2+/-0.2%, which compares very well with the density changes. The corresponding XRD values are determined to be 6.43-6.48 and 6.48 g/cm(3) for NaCl-type and the hexagonal phases, respectively. This shows that nearly void-free films are formed.