Abstract We demonstrate a good correspondence of the results obtained by PICTS and by fitting isothermal current transients when a multiexponential form with well separated exponentials can be used for a description of the transients and at the same time the PICTS spectra give a linear Arrhenius plot. The results are also supported by TSC measurements below the room temperature. All the measurements have been performed on samples with a three electrode configuration which makes it possible to separate surface and bulk related signals. It has turned out that the negative peak observed on our samples in the PICTS spectra above the room temperature cannot be explained by surface related phenomena only. This is to our knowledge the first direct experimental proof of the nature of the negative peak which has recently been a subject of discussion in the literature.