For several years, the Beta Secondary Standard (BSS 2) developed at PTB has been in worldwide use to perform irradiations with calibrated beta sources. In this work, the photon contamination of the reference beta radiation fields was measured using a high purity germanium (HPGe) spectrometer. Both the spectral distribution of the photons and the resulting dose(rate) was deduced from the measurements. The dose due to photons turned out to be about 0.6% for 147 Pm sources, 0.5% for 106 Ru/ 106 Rh sources, and less than 0.1% for 85 Kr and 90 Sr/ 90 Y sources compared to the dose due to betas. For 147 Pm, 85 Kr, and 90 Sr/ 90 Y sources, the dominating contribution consists of bremsstrahlung and characteristic X-rays produced in the source encapsulation, while for 106 Ru/ 106 Rh sources the gamma emissions from the radionuclides themselves are dominant.