The self-diffusion coefficients (D) of n-C21H44 in long channels of deuterated urea-d(4)/n-C21H44 adduct have been successfully measured over a wide range of temperatures from 0 to 75 degreesC by means of high field-gradient spin-echo H-1 NMR method, in order to clarify diffusional behavior of the n-paraffin. From these experimental results, it is found that n-C21H44 molecules are diffusing in long urea channels of urea adduct and has two diffusion components such as the fast diffusion component (D approximate to 10(-10) m(2)/s) and the slow diffusion component (D approximate to 10(-12) m(2)/s), and that n-C21H44 molecules at the two external regions near the ends of the channel and the inner region in one of long channels of urea adduct contribute to the fast diffusion component and the slow diffusion component, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.