The alpha relaxation of a series of ether-ketone model compounds was studied to quantify the effect of molecular stiffness on the Kohlrausch-Williams-Watts stretching exponential. The obtained values decrease with increasing stiffness and are within the theoretical bounds predicted for systems with spatial dimensionality varying from 2 to 1. (C) 2000 American Institute of Physics. [S0021-9606(00)51512-8].