The ion adsorption/desorption processes initiated by LC filling in liquid crystal (LC) cells are studied. The LC we used was 5CB and the aligning substrates were UV irradiated polyvinylcinnamate (PVCN) films providing planar alignment of LC. It is shown that intensity of adsorption/desorption processes is strongly determined by purity of LC and irradiation associated parameters, such as exposure dose tau(exp) and time period between irradiation and LC filling tau(f-i). To fill the cells we used both 5CB kept in air and 5CB vacuumed at 10(-2) Torr over 1 h. The ion concentration in first portion of LC was estimated to be higher by factor 2 compared with the second one. The cell filling with LC kept in atmosphere is associated with intensive ion adsorption from LC bulk to aligning substrates (when exposure time is zero or when tau(f-1)>1h ) or with both adsorption and desorption of ions (in case when tau(f-i) << 1h). In contrast, cell filling with vacuumed LC mainly initiates ions desorption from the polymer substrates. The latter effect is observed for non-irradiated cells too. This shows that even non-irradiated substrate charges, more likely by adsorption of air ions. The following UV irradiation just increases charging of aligning substrates.