The relationship between number average molecular weight (M-n) and intrinsic viscosity ([eta]) was studied for poly(1,4-butylene adipate) diol (PBAD) in tetrahydrofuran, toluene, and ethyl acetate at 25 degrees C. Thus, a series of PBAD samples were prepared by polymerization between 1,6-adipic acid and 1,4-butanediol. The values of M-n for the samples were determined by end-group analysis as well as by ebulliometry, and the average difference of M-n between the two analysis ways was about 2.69%. The Mark Houwink Sakurada equations for PBAD were obtained to relate [eta] with M-n in the range of 1900-10,000. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 1883-1887, 2010