Continuous wave and pulsed optical beam induced current measurements were carried out onn‐InP/n‐InGaAs/n‐InP double heterostructures which contain large areas of diffused InGaAsp‐njunctions. The flat cw photocurrent and illumination‐position independence of the transient photocurrent response, observed when the optical source spot illuminates a floating diode, demonstrates the effect of photocarrier spreading in ap‐njunction. The very long photocarrier spreading length observed at low optical power may be an important parasitic coupling mechanism which should be considered in the isolation‐design of integrated optoelectronic devices.