Amorphous indium oxide (a:InO) plays a prominent role in the study of strongly disordered superconductors. In particular, the disorder-driven transition (SIT) between superconducting and insulating states can be realized. Compared to the superconducting side of the SIT, the insulating side has been explored much less experimentally due to the lack of appropriate experimental means. Here we present dielectric measurements on insulating a:InO, performed at GHz frequencies and at temperatures down to the mK regime, on a set of samples with varying disorder. We obtain the real and imaginary parts of the dielectric function (corresponding to frequency-dependent conductivity) as function of disorder, temperature, and frequency. We analyse these data based on theory for hopping in disordered systems, and we trace the evolution of the dielectric function, e.g. the increase of its real part upon approaching the SIT.