To help ascertain the contribution semifusinite makes to plasticity development during carbonization, semifusinite and vitrinite fractions with purities over 90% have been obtained by density gradient centrifugation from a medium-volatile Australian bituminous coal. In-situ H-1 NMR, during which the maceral concentrates were heated to 550 degrees C, has been used to determine the amount and mobility of fluid material developed. At maximum fluidity, the vitrinite fractions generate similar proportions of fluid material as the whole coal, accounting for 30% of the hydrogen, while the semifusinite fractions yield only ca. 15% mobile hydrogen. The mobility of the fluid material is significantly higher for the vitrinite concentrates than for both the whole coal and the semifusinite fractions (T-2＇s of over 70 mu s cf. 52 and 45 mu s, respectively). For the vitrinite concentrates, the increases in reflectance and the degree of aromatic ring condensation with density correlate with the decreasing mobility of the fluid material. For the coal investigated, the overall contribution of the reactive semifusinite to the fluid phase at maximum fluidity is estimated to be 15% (ca. 4 of the 30% mobile hydrogen observed). Furthermore, there is a reasonably good linear relationship (R-2 > 0.96) between fluidity (for both the proportion of the fluid material and its mobility or T-2) and density for all the maceral concentrates investigated, and therefore, samples,vith very similar petrographic composition can exhibit significantly different fluidity characteristics. Although interactions between maceral groups may take place in the original coal matrix, mixtures of the vitrinite and semifusinite concentratres did not reveal any synergism.