The results of two experiments designed to measure the concentration (and, in one case, the relevant mobilities) of charged particles of both signs in the mesosphere region are presented. In the mobility experiments two groups of positively charged particles were found in the height region 58 to 72 km. The heavy group was about four times less, the light group about eight times more mobile than the extrapolated mobility of light ions measured on the ground. The mobility of the negative particles was about equal to the mobility of the light group of positive carriers. It was therefore concluded that the negative ions and the ‘light’ positive carriers were molecule ions, whereas the heavy, positively charged group belonged to heavier, aerosol particles. The second experiment, a modified Langmuir probe arrangement, was designed to determine the critical height above which the probe current is predominantly caused by free electrons. This height was found to be around 62–63 km. In both experiments a considerable increase of ionization was found below 50 km, whereas two other flights with the modified Langmuir probe did not show this additional layer. It was found that both the mobility experiment and the Langmuir experiment were flown about half an hour after an SID, whereas the other two Langmuir experiments were flown under normal conditions. It is suspected that the observed additional ionization was generated in connection with the SID events by either wave or particle radiation.