The free volume of a bisphenol-A-type epoxide oligomer (DGEBA) was studied using Williams-Landel-Ferry parameters and thermal expansion coefficients above and below the glass transition temperature (T-g). The values of the free-volume fraction at the T-g are around 0.02 for the DGEBA oligomers having weight-average molecular weights ((M) over bar(w)＇s) from 1396 to 2640. The dipole mobility, which was obtained from the analysis of the temperature dependence of the dielectric relaxation time, was compared with the segment mobility in terms of the critical volume for the transport of each moving unit. The critical volume for the segment transport increases with increase of the M, of the oligomer. The critical volume for the dipole movement, on the other hand, is not different between the oligomers studied (1396 less than or equal to (M) over bar(w) less than or equal to 2640), which leads to that the dipole mobility in the epoxide oligomer is smaller than is the segment mobility. The low mobility of the dipole is considered to result from the molecular interaction restricting the dipole movement, especially in a smaller (M) over bar(w) oligomer.