A sharplike modulation of the linear electrooptic (LEO) coefficient with pressure (p) and temperature (T) has been observed for the first time. The modulation depth of the LEO (r(222)) tenser component in the C-60-TTF (for the He-Ne laser, lambda = 633 nm) was equal to about 2.6%. By varying the applied hydrostatic pressure to up to 15 GPa and the temperature from 4.2 to 25 K, we have unambiguously demonstrated that the observed effect is caused by the incorporation of the TTF moiety. This effect is absent from the unsubstituted fullerenes when the applied hydrostatic pressure is increased to 19 GPa and the temperature varied from 4.2 to 150 K. Norm-conserving self-consistent pseudopotential and molecular dynamics simulations independently reproduce these modulations and attribute them to the electron-vibration interactions associated with the LEO coefficients. The contribution of the interconformational tunneling under the applied pressure and temperature is demonstrated. The data obtained give a possibility for using the discovered effect for contactless measurements of low temperatures and high pressures.