The thin SiC filament technique has been employed to identify the possibility of measuring flame temperature, and especially the inner flame temperature, by comparing the relative visible (non-IR) luminosities of SiC filaments with temperature measured in coflowing, laminar propane/air diffusion flames. The results show good agreement between the digitized relative visible luminosity profiles of the SiC filaments and temperature profiles measured using a thermocouple at temperatures above 700 C, although a nonlinear calibration is probably required for the whole temperature range. In investigating soot inception in a laminar diffusion flame, it was found that a sharply decreasing region of temperature exists in a narrow region of the sharply increasing soot luminous intensity zone, which supports the proposition that the dominant reaction in the soot inception zone is endothermic in character. The estimated inner flame temperature gradient from the SiC filament luminosity-determined temperature profile was 400degreesC-600degreesC/mm, and is similar to the outer flame temperature gradients, but is steeper than those of earlier studies. Simultaneously, the highest radial peak temperature exists near the nozzle exit, and the centerline temperatures were virtually unchanged with increasing flame height in an oxidizer-deficient near-extinction condition. Soot deposition is considerably decreased with decreasing coflowing airflow rates. These suggest that a nonsooting blue reaction flame caused by pyrolysis is dominant, and that the major portion of the heat-release reaction is performed in the flame surface near the nozzle exit in an oxidizer-deficient ambient.