Temperature dependent powder and single-crystal synchrotron diffraction, specific heat, magnetic susceptibility and small-angle neutron scattering experiments have revealed an anomalous response of MnGe. The anomaly becomes smeared out with decreasing Mn content in Mn_1-xCo_xGe and Mn_1-xFe_xGe solid solutions. Mn spin state instability is discussed as a possible candidate for the observed effects.