The flowing solvent liquefaction of a brown coal was performed at temperatures from 623 to 708 K and a pressure of 10 MPa aiming at experimental definition of the primary conversion, which was measured in terms of the mass yield, X-f, of products dissolved into flowing tetralin or 1-methylnaphthatene as a function of temperature and holding time. In-situ isolation of the products from the solid residue by a forced flow of the solvent at a sufficient rate enabled the definition of X-f as the primary conversion. At each temperature, X-f using tetralin increased with time and leveled off at a conversion, X-fm of 0.96 which was independent of the temperature in a range from 673 to 708 K. The results further revealed that that the intraparticle solvent-to-coal hydrogen transfer enhances the primary conversion. Batch liquefaction experiments were also performed at 623 and 693 K using tetralin in the presence or absence of an iron/sulfur catalyst under 10.1 MPa hydrogen gas atmosphere, and the conversion of the coal, X-b, was determined by a common procedure as the mass yield of product soluble to extraction solvent, tetrahydrofuran (THF). X-b in the non-catalytic liquefaction was appreciably lower than X-f at 623 K, since THF cannot dissolve entirely the product that was dissolved into tetralin during the liquefaction. Due to extensive degradation of the primary product dissolved into tetralin into the catalytic liquefaction at 623 and 693 K and even in non-catalytic one at 693 K, the dissolved product converted into THF-soluble product with molecular mass lower than 2,000, allowing X-b to be equivalent to X-fm.