The aim of this study is to measure the diffusion coefficients of pyridine in Turkish Cayirhan lignite (C: 57.1 wt %, dmmf) at temperatures about 20-27 degrees C and determine the type of transport mechanism of diffusion. The raw coal sample was demineralized with HCl and HF by standard methods, and the raw and demineralized coal samples were extracted with pyridine. To investigate the diffusion of pyridine vapor in coal samples, the mass of pyridine uptake per mass of coal sample (M-t/M-infinity) was calculated as a function of time. The diffusion coefficients were measured from the slope of graphs of M-t/M-infinity versus t(1/2). The diffusion coefficient of pyridine in the raw coal increased from 10.0 x 10(-15) to 11.9 x 10(-15) m(2)/s when the temperature was elevated from 21.1 to 26.9 degrees C, respectively. The diffusion coefficients of pyridine raw coal and of those treated with HCl and HF were 11.9 x 10(-15), 4.3 x 10(-15), and 4.8 x 10(-15) m(2)/s at 26.9 degrees C, respectively. The studies in the present work on pyridine vapor diffusion in raw coals have generally indicated that the diffusion obeyed the Fickian diffusion mechanism the temperatures 20.0-27.0 degrees C. Generally, the diffusion exponent values increased when the temperature elevated from 20.0 to 27.0 degrees C, but this raise placed the diffusion of pyridine between the Fickian diffusion and Case II diffusion mechanisms.