The present paper is a trial to shed further light on the dependence of the junction characteristics of the proposed optocoupler type 4N25 on the operating conditions; the applied voltage and signal frequency. For the input light emitting diode and output phototransistor, their forward and reverse (C–V) characteristics were investigated and plotted at different applied bias voltage- and frequency- levels. In this concern, the diffusion- and transition- capacitances, impedance, quality- and dissipation- factors, and the phase angle were investigated. Concerning the light emitting diode, at frequency value of 200 kHz, as an example, the diffusion capacitance value increases exponentially from 0.166 nF, up to 4.71 nF, measured at forward bias voltages of 0.01 Volt and 1.20 Volts, respectively. Also, at an applied forward bias voltage of 1.0 Volts, the diffusion capacitance increases from 1.76 nF up to 2.84 nF, measured at frequency levels of 50 kHz and 300 kHz, respectively. Considering the transition capacitance, and at signal frequency of 200 kHz, its value was shown to be decreased rapidly from 178.36 pF down to 30.42 pF whenever measured at 0.01 Volt and 0.30 Volt, respectively. But for higher bias voltages, the decreasing rate was shown to be negligible. On the other hand, at applied bias voltage of 1.0 Volt, its value was shown to be decreased from 28.76 pF down to 17.59 pF, measured at frequency levels of 50 kHz and 300 kHz.  For the phototransistor, its emitter junction capacitance increased exponentially as a function of the emitter-base bias voltage, where at frequency of 200 kHz, as an example, its value increased from 0.581 nF up to 3.048 nF, measured at 0.01 Volt and 0.80 Volt, respectively. On the other hand, the collector junction capacitance was shown to be decayed rapidly as a function of the applied reverse bias voltage. Where its value decreased from 1.29 nF down to 0.012 nF, measured at 0.01 Volt and 0.90 Volt, at frequency of 200 kHz, as an example. Finally, it is proved that for both capacitances, their values are a direct decreasing functions of the operating frequency, where at applied bias voltage of 0.8 Volt, a values of 7.6878 nF and 0.05338 nF were decreased down to 1.811 nF and 0.0254 nF, respectively, with increasing the frequency from 50 kHz up to 300 kHz.