The 3C-6H polytypic transition in SiC single crystals is studied by means of diffuse x-ray scattering. Based on numerical simulations of the diffuse scattering intensity distribution we unambiguously prove that the 3C-6H transition in SiC occurs through the glide of partial dislocations and not by the "layer displacement" mechanism ͑i.e., local diffusional rearrangement of the Si and C atoms͒. The technique is extremely sensitive and can be used as a nondestructive mean to obtain statistically relevant values of the transition level down to ϳ0.05%.