The relationships between free radical concentration (N-g) and vitrinite reflectance (%R-o) of coal were tested through 36 laboratory anhydrous pyrolysis experiments. The experiments were carried out at isothermal reactor with pyrolysis temperatures ranging from 300 to 550 degrees C over 30-480 min. The effects of temperature and retention time on the Ng were investigated. The concentrations of free radicals in pyrolysis coal samples increase with increasing time and temperature of experiments and reach a maximum value at about 1.75 %R, and then decrease with further maturation. The Temperature Time Index (TTI) integral equation was applied to establish the relationship between the TTI and N-g, in this paper. As the free radical concentration in organic matter is strongly maturity dependent, it may prove to be a valuable geothermometer for paleogeotemperature study in sedimentary basins. (c) 2006 Elsevier B.V All rights reserved.