The concentration dependence of the self-diffusion coefficient of monodisperse poly(ethylene oxide) (PEG) dissolved in deuterated water has been measured for three polymer samples. Two samples have the same nominal molecular weight M(n) approximate to 10000 but differ in molecular architecture-linear and cyclic-while the third is a smaller, linear sample for comparison. The concentration behavior of the self-diffusion coefficient for the cyclic sample suggests a much smaller hydrodynamic radius compared to the linear sample of equivalent molecular weight. From a comparison of the two linear samples, an estimate of the "equivalent linear" molecular weight of the cyclic polymer can be obtained.