Nanoscopic metal fluorides with surface hydroxy groups are of broad interest in heterogeneous catalysis. With both Lewis and Brønsted acid sites on the surface, these catalysts can be applied to a wide range of reactions. Having previously synthesised AlF3- and MgF2-based catalysts, we report a new transition metal fluoride with bi-acidity. A pre-dehydration procedure was developed to introduce hydroxy groups to a Lewis acid, FeF3. Subsequently, ternary nanoscopic FeF3-MgF2 with enhanced porosity was prepared through a one-step fluorination. The interaction between MgF2 and FeF3 was elucidated. Surface characterisation revealed a remarkable increase in the surface area of FeF3-MgF2 compared with FeF3. More importantly, medium–strong Lewis and Brønsted acid sites were detected on the FeF3-MgF2 surface. In line with its bi-acidity, FeF3-MgF2 was highly active in the model reaction, the isomerisation of citronellal to isopulegol. Finally, we discuss how the porosity and surface acidity jointly determine the activity of FeF3-MgF2. Our study demonstrates the feasibility of ternary FeF3-MgF2 and opens new possibilities to synthesise bi-acidic fluoride catalysts.