Thermal/optical methods are widely used in the determination of aerosol organic carbon (OC) and elemental carbon (EC) collected on quartz filters. A fraction of OC undergoes charring to form pyrolytically generated EC (PEC) during thermal analysis. The correct speciation of OC and EC in thermaVoptical methods depends on one of the following two assumptions: (1) PEC evolves before native EC evolves in the analysis or (2) PEC and native EC have the same apparent light absorption coefficient (sigma) at the monitoring light wavelength. Neither of these assumptions has actually ever been checked or tested. The first assumption is invalidated by the observation that the combustion of PEC overlaps that of native EC despite multiple stepwise combustion at temperatures ranging from 575 to 910 degrees C. An examination of sigma versus EC evolution indicates that the sigma values of PEC and EC are not the same in most cases and the a value of PEC is not constant during a single thermal analysis. The second assumption is thus invalid as well. The measured EC concentrations can either overestimate or underestimate the true native EC concentrations depending on the relative magnitude of the a values of the PEC and native EC at the point where the instrument sets the EC/OC split line. Both over- and underestimation have been observed in real aerosol samples. The unequal a values of PEC and EC also explain that different temperature programs, when employed to analyze the same filter samples, systematically yield different EC and OC concentrations. Our findings imply that minimizing charring improves the accuracy of the EC/OC split in thermal/optical methods.