Photon beam collimators are a source of secondary electrons that contaminate the beams and increase the surface dose. Calculations and measurements of this contamination have been made for 6 MV and 21 MV X-ray beams. Calculations are made using an electron transport model including the production of electrons by the photons and their transport and multiple scatter, both in the collimator and in the air. The variation with collimator material and geometry is investigated. The lowest contamination is obtained for high density and atomic numbers because of the small lateral electron range and large linear stopping power. The collimator geometry is also very important. If the collimator surfaces are hit by the photons, the collimator-produced electrons reaching the phantom may be increased by a factor of 2 or more. Calculations show that the surface dose from these electrons is less than 5% for 21 MV X-rays. This means that the dominating source of contaminating electrons at this energy is the beam flattening filter.