We describe a new method and gel probe for measuring photocurrent action spectra of cuprous oxide thin films, without the need for assembling solar cells or using a liquid electrolyte cell. Thus, external quantum efficiency of photoactive films can be measured directly without coating a transparent conducting oxide layer or using metal evaporation for contacts. With an aqueous gel, we observe dissolution of the CuO and Cu(2)O layers, causing photocurrent decay. Replacement of the aqueous gel electrolyte with a non-aqueous gel electrolyte significantly stabilized photocurrents. A mechanism is proposed to account for all the electrochemical observations. (C) 2010 Elsevier B.V. All rights reserved.