From a series of in situ photoemission experiments macroscopic electric fields are clearly demonstrated in SiC/AlN, SiC/GaN, and GaN/AlN heterostructures grown by molecular beam epitaxy on 6H-SiC(0001). A significant contribution is due to the spontaneous polarization; the piezoelectric term alone would not explain the sign of the field measured in SiC/AlN. The experimental field has lower intensity as compared to theory: the role of electronic gap slates at the surface is pointed out. A self-consistent tight-binding approach which is able to describe polarization fields, dielectric screening, and free carrier screening is applied for a more consistent theoretical discussion of the experimental data. The valence band offset (VBO) has been determined for all heterojunctions under study and the apparent dependence on the overlayer thickness, due to the presence of the strong polarization fields, has been pointed out in view of a correct determination of the VBO. The VBOs at the heterojunctions obtained by extrapolation to zero overlayer thickness are (substrate/overlayer) : [(1.5-1.7)+/-0.1] eV for SiC/AlN, [(0.7-0.9)+/-0.1] eV for SiC/GaN, (-0.3+/-0.1) eV for AlN/GaN, and [(0.15-0.4)+/-0.1] eV for GaN/AlN.