A variable-force-loading needle penetrometry and a proton magnetic resonance analysis were performed for in situ measurements of shear-rate-independent viscosity of softening coal pellet upon heating, eta, and the fraction of mobile hydrogen existing in the liquid phase, phi(mh), respectively. During isothermal heating of the pellet at temperature in a range from 680 to 730 K, phi(mh) changed with time via a maximum while eta did inversely. At every temperature examined, the time for the maximum phi(mh) coincided with that for the minimum eta. This result qualitatively validated the experimental definition of the liquid fraction in the softening coal as a liquid/solid suspension by 0 h. Further analysis of the results revealed that the logarithm of eta, which changes in a range from 10(10) to 10(4) Pa s upon isothermal heating, is correlated linearly with the liquid fraction ranging from 0.1 to 0.5. For each of the pellets made of two different coals, it was found that the logarithm eta and phi(mh) varied being governed by a single linear relationship upon both isothermal heating and non-isothermal heating. Such a single relationship, which was valid over a temperature range from 600 to 800 K, suggested fairly small temperature dependency of the viscosity of liquid in the softening coal. (C) 2003 Elsevier Science Ltd. All rights reserved.