The preparative flexibility of hydrothermal syntheses needs to be systemised for exploring complex structure-synthesis relationships and morphology control options in materials chemistry. This is demonstrated for the targeted hydrothermal preparation of molybdenum oxide materials: firstly, in situ studies were employed for the efficient production of MoO(3) nanofibres. Furthermore, ionic substances as structure-directing tools brought forward a new class of fluorinated polyoxomolybdates.