The collimator scatter factor Sc has generally been measured in air using an ionization chamber inside a buildup cap or mini-phantom. Here, Sc was measured in phantom at 10 cm depth for 6 and 15 MV photons with square collimator settings of 2.5-40.0 cm. The results were consistent with in air measurements with a mini-phantom to within 0.4%. In the measurements, a series of Cerrobend field shaping blocks were used to define the field size in the phantom while the collimator settings were varied from the field size in the phantom to twice that value. Corrections of up to 2% for scattered radiation from the added Cerrobend field shaping blocks were necessary. Since a buildup cap or mini-phantom is not used, the smallest field size that can be measured is limited only by the size of the detector and the measurement is performed with full scatter resembling the treatment condition of a patient.