The existence of post-effects in aqueous solutions after plasma treatment by glow discharge and by diaphragm discharge at atmospheric pressure was the subject of our investigations. Organic dyes, phenol, compounds of Cr3+, Mn7+ were under our study. It was revealed that processes taking place in solution under plasma treatment proceed while activated by discharge electrolyte being added to the starting solution. The existence of the post-effect and its magnitude were found to be in dependence of different factors, i.e. from the type of discharge, the time of activation, the time between the treatment and the use of the solution, properties and concentration of dissolved substance, pH of solution.