K 2 Ge 8 O 17 shows a structural phase transition near T c ≈ 269 K. The temperature dependence of the optical birefringence n, of the elastic stiffness constants c ij and of the effective electro-optical coefficients r (eff ) is determined. At T c the derivative ∂ n/∂T changes its sign. Elastic properties are studied by recording the acoustic resonances of a rectangular parallelepiped. We discuss those features of the method which are important in the investigation of phase transitions. No signature of elastic softening near T c is observed in K 2 Ge 8 O 17 . Below T c , c 44 and c 66 increase continuously. The elastic hardening is accompanied by the onset of a small electro-optical effect. Its existence demonstrates the loss of the centre of symmetry, its anisotropy is consistent with a triclinic symmetry, and its magnitude suggests a spontaneous polarization of about 6 × 10 −4 C m −2 in the low-temperature phase. Although K 2 Ge 8 O 17 is reminiscent of a weak ferroelectric material there are some observations which are uncommon to a paraelectric-ferroelectric phase transition.