Abstract The main purpose of the present study is to correlate the presence of controlled amounts of residual OH groups with the values of several physical properties, namely, the characteristic temperatures, linear thermal expansion coefficient, density, refractive index, microhardness, ultrasonic velocities, elastic constants and infrared transmission of ZrF 4-based fluoride glasses. A series of glass samples were prepared in the ZrF 4BaF 2LaF 3AlF 3NaF system, with increasing NaF contents. Batches for the reference (dry) glass samples were first dried in a vacuum oven and were then melted in Pt crucibles in a dry box filled with high purity N 2. Weakly hydrated glasses were prepared according to a similar procedure, but the batches were kept in water vapor saturated atmosphere for at least 24 h before fusion. Strongly hydrated glasses were melted in a fume hood, under a water vapor saturated stream of N 2. For levels of OH incorporation up to 8 × 10 −4 mol l −1 for ZBLA, 1.0 × 10 −3 mol l −1 for ZBLAN 6.6 and 1.5 × 10 −3 mol l −1 for ZBLAN 17.8, the density, refractive index and glass transition temperature, T g, decrease with increasing OH content (by 0.8%, 0.25% and 3.0%, respectively) is similar to what has been observed for v-SiO 2. At OH levels greater than 8 × 10 −4 mol l −1 for ZBLA, 1.0 × 10 −3 mol l −1 for ZBLAN 6.6 and 1.5 × 10 −3 mol l −1 for ZBLAN 17.8, an increase in physical properties, such as the density, refractive index and T g with increasing OH content, was noticed (of 0.8%, 0.4% and 2.8%, respectively), similar to what has been observed in silicate glasses. This behavior strongly suggests the presence of hydrogen bonded ZrOH species. A structural model is proposed for a typical fluorozirconate ZBLAN glass, based on structural data and physico-chemical properties. The model allows a correlation between the effect of residual OH groups and the values of some of the most representative properties of these glasses.