Thus far the many reports concerning the utilization of the photocarrier grating (PCG) method have assumed that ambipolar transport takes place in such a PCG when it is imposed on hydrogenated amorphous silicon (a‐Si:H). This assumption, which is decisive in the interpretation of the experimental results in terms of the ambipolar diffusion length, has not been tested thus far. In this letter a corresponding testing criterion is proposed, and it is demonstrated that whenever ambipolarity is lost, the PCG‐derived diffusion lengths may be wrong. The finding that ambipolarity is maintained in device qualitya‐Si:H is shown to confirm the theoretical suggestion that, whenever observed, the ambipolarity ina‐Si:H is due to shallow trapping effects.