A series of semi-cokes, containing from virtually 0 to 100% mesophase, were produced from the THF-soluble fraction of a soft coal tar pitch. The mesophase contents were estimated by high temperature in situ H-1 NMR and optical microscopy with extremely close agreement being obtained between the two techniques. Quantitative solid state C-13 NMR measurements using the single pulse excitation technique indicated that the extent of anisotropy in the pitch is a function of the condensation of the aromatic structure, where isotropic pitches contain 5-6 rings on average, assuming pericondensation, which increases to 9-10 rings for the fully converted mesophase pitches. Characterisation of the pyridine-solubles (PS) and insolubles (PI) from the pitches confirmed that development of mesophase in the coal tar pitch is limited by the loss of low molecular mass compounds, in that the PS contain 5-6 rings throughout the series while the PI, even in largely the initial isotropic state, are as condensed as in the anisotropic pitches. (C) 1998 Elsevier Science Ltd. All rights reserved.