Plastic organic scintillators are often considered 'water equivalent' in megavoltage photon beams as they have similar atomic composition and density to water. Two plastic scintillators are evaluated for their 'water equivalence', using the Burlin cavity theory, in the photon energy range of 200 keV to 20 MeV. The Burlin theory predicts that the two investigated scintillator materials are likely to be 'water equivalent' in the energy range investigated. The prediction of the Burlin theory for one of these scintillators has been confirmed by absorbed dose measurements in bremsstrahlung beams between 13 and 19 MeV.