The conventional technique for the measurement of mobility sandwiches the sample between two electrodes, at least one of which is transparent. The sample is illuminated through this surface and the resultant current is measured under constant voltage conditions. In the present work we describe a modified method for the determination of mobility where one surface of the sample is grounded and the other is left floating. The floating surface is charged to an initial voltage,V0and its subsequent decay upon exposure to intense light is observed experimentally. The relation between the instantaneous surface voltageV(t) and the exposure timetis referred to as the ``photoinduced discharge characteristic'' (PIDC) of the photoconductor plate. Knowing the PIDC and the theoretical expressions derived in this paper one can obtain an accurate value for the mobility of the charge carriers. As verification of our analysis we have measured the PIDC of amorphous Se films deposited onto Al substrates. Both the surface potential and its time rate of decay were measured simultaneously through a NESA surface potential probe. The agreement between theory and experiment is excellent and the value of the hole mobility obtained using our analysis is in good agreement with those already reported in the literature.