In this study of Zn‐diffused red‐emitting diodes, measurements of emission spectra,I‐Vcharacteristics, electroluminescence (EL) saturation, electron diffusion lengths, and photocapacitance have been compiled. Two factors which contribute to the observed low EL efficiency of diffused diodes have been identified. One is the presence of deep level centers, as indicated by large space‐charge recombination currents. The other is a relatively small product of the Zn&sngbnd;O pair concentration and the electron diffusion length, which determines the EL level at Zn&sngbnd;O pair saturation. Results from photocapacitance measurements indicate that for diffused diodes, the oxygen, and hence the pair concentration, is smaller by a factor of 2–3 than that found in double liquid‐phase epitaxy (LPE) diodes. The effects of an optimized annealing procedure (750°C for 8 h, followed by 525°C for 16 h) were also identified. The 750°C anneal primarily reduces the deep state concentration, whereas the 525°C annealing results in increased Zn&sngbnd;O pair concentrations.