Morus nigra L. (black mulberry) and Morus alba L. (white mulberry) display high concentrations of health-promoting compounds, particularly phenolics. However, no published studies have addressed the changes in the content of phenolic compounds during frozen storage, a widely used form of preservation of these fruit in the Turkish countryside. This work was undertaken to determine these alterations, if any, in order to assess whether the bioactive properties of the produce may be altered significantly. Materials and methods. Black and white mulberry fruit were collected at commercial maturity and frozen at -25 ºC for up to 5 months. The content of selected phenolic acids and flavonoids was analysed at harvest on fresh fruit and at monthly intervals on thawed samples by High-Performance Liquid Chromatography with Diode-Array Detection (HPLC/DAD). Results and discussion. Phenolic compound levels were higher in black than in white mulberry fruit at harvest. Rutin and chlorogenic acid predominated quantitatively in black mulberry, and decreased along frozen storage even though some fluctuations were observed. Cathechin was the main compound detected in white mulberry, and remained largely stable during the whole experimental period. Conclusion. Although the concentration of the investigated phenolics varied to different extents during frozen storage, fruit retained acceptable levels, which suggests that this practice allows preserving satisfactorily the health-promoting properties which characterise these fruit species.