The storage phosphor plate (SPP) is a reusable radiation image detector, widely used in diagnostic computed radiography, x‐ray crystallography and radioactive tracer studies. When exposed to ionizing radiation, the SPP stores a latent image until it is scanned with a red reading laser which causes blue photostimulated luminescent (PSL) photons to be emitted. The mechanism of formation of the latent image is still poorly understood, especially for megavoltage photon beams. In order to gain insight into this mechanism and aid applications to high‐energy beam dosimetry, the authors have directly determined the SPP generation efficiency, , the energy required to produce one quantum of emitted PSL when it is irradiated by and photon beams. This was done in four steps: 1. The SPP, in a water‐equivalent plastic (WEP) phantom, was exposed to a or beam, which had been calibrated to give a known absorbed dose to water in a water phantom at the position of the sensitive layer of the SPP. 2. Monte Carlo simulations were used to calculate the ratio of the dose to the sensitive layer in the WEP phantom to the dose to water at the same position in a water phantom. 3. A bleaching experiment was used to determine the number of photons emitted by a plate given a known dose. 4. The generation efficiency was calculated from the number of photons and the dose. This method is much more direct than previous calculations for kilovoltage x‐ray beams based on quantum noise analysis. W was found, within experimental uncertainty, to be for and for , independent of dose. The values for kilovoltage x‐ray beams determined previously agree, within their large uncertainty, with these values for megavoltage beams.