Observations have been made of current (0.1 to 10,000 &mgr;a) in a Weston Photronic blocking‐layer photo‐cell, as a function of illumination (1 to 100,000 lux) and of potential difference across the cell. The use of a graph of current against voltage is discussed, as well as the conditions for maximum external power (largest observed value about 1 milliwatt). The leakage current through the blocking layer is calculated and from it the d.c. conductance in the low resistance direction is obtained. This is found to consist of a constant term of the order of 80 micromhos, as well as terms proportional to the 0.8 power of the leakage current and the 0.4 power of the illumination. A fourth term is ascribed to the effect of the potential across the blocking layer in preventing the passage of low velocity photoelectrons across the layer. With maximum illumination a conductance as high as 9000 micromhos is obtained. This rises to 30,000 micromhos or more with the largest leakage currents. A Westinghouse Photox cell was found to possess an internal conductance which changes very little with current, and which contains a term varying as the 0.4 power of the illumination. Comparison of the two cells indicates that nonlinearity of the current‐voltage characteristic and rectification (almost absent and becoming reversed in the Photox cell) are not essentials in blocking‐layer photo‐cells. The open circuit voltages of both cells are found to be nearly proportional to the illumination for voltages less than 5 mv and to be proportional to the logarithm of the illumination for voltages greater than 50 mv.