A theory is presented for the onset of shearthickening in colloidal suspensions of particles, stabilizedby an electrostatic repulsion. Based on an activation model,a critical shear stress can be derived for the onset of shearthickening in dense suspensions for a constant potential anda constant charge approach of the spheres. Unlike previousmodels, the total interaction potential is taken into account(sum of attraction and repulsion). The critical shear stress isrelated to the maximum of the total interaction potentialscaled by the free volume per particle. A comparison withexperimental investigations shows the applicability of thetheory.