Photon correlation spectroscopy was employed in the polarized geometry in order to investigate the dynamics of solutions of substituted poly(p-phenylenes) at high concentrations. The total concentration fluctuations were found to exhibit a two-step decay. The fast dominant relaxation was the classical cooperative diffusion, controlled by the osmotic pressure of the system, exhibiting a stronger concentration dependence compared to flexible polymers, and discussed in the framework of the wormlike model. On the other hand, the second diffusive process was not related to self-diffusion (as confirmed by independent pulsed field gradient NMR measurements) and could not be accounted for by the existing theories. The self-friction coefficient was found to exhibit a stronger concentration dependence than the cooperative friction coefficient. At high concentrations, a diffusive cluster process was detected, despite the high solubility of these polymers in different solvents. The characteristics of this mode depended strongly on the molecular weight.