The evolution of HCl during pyrolysis of 16 coals with different ranks at a heating rate of 10 degrees C/min has been studied with an online monitoring method. Approximately 50%-95% of total chlorine is converted to HCl up to 800 degrees C, and the remainder is mostly retained in the char, which leads to a strong reverse correlation between the two. As the sum of Na and Ca naturally present in coal increases, the amount of HCl tends to decrease. The temperature dependence of the rate of HCl evolved differs with each coal and shows at least four peaks at 280, 360, 480, and 580 degrees C. The former two peaks are present for two coals alone, whereas the higher temperature HCl formation at :450 degrees C is common for almost all of the coals. The HCl peaks at 280 and 360 degrees C are considerably small by water washing. When model chlorine compounds added to activated carbon, such as hydrated NaCl, hydrated CaCl2, and organic hydrochlorides, are pyrolyzed in the same manner as above, HCl formation occurs dominantly between 250 and 450 degrees C in every case. The pretreatment of a brown coal char with HCl at 500 degrees C and subsequent temperature-programmed desorption (TPD) measurement up to 950 T suggest that HCl reacts with the nascent char upon pretreatment to form several types of Cl functional forms, from which the HCl desorption takes place at 450-750 degrees C upon TPD. The HCl evolved at < 450 degrees C during pyrolysis may arise from water-soluble Cl functional groups in coal, whereas the HCl formation at 450 degrees C observed for almost all of the coals may proceed through a mechanism involving secondary reactions of HCl evolved at a lower temperature.