The diquark model in which protons are viewed as being built up by quarks and diquarks (the latter are treated as quasi-elementary constituents at moderately large momentum transfer) is applied to Compton scattering off protons and combined with similar studies of the electromagnetic form factors of nucleons and of [Formula: see text]. It is demonstrated that the model is able to describe very well the data for these reactions at moderately large values of the momentum transfer with a common set of parameters. We also investigate the helicity dependence of the amplitudes. Very interesting and characteristic predictions for various observables are given. In particular a sizeable transversal polarization of the proton is obtained for Compton scattering.