The dimensionless extinction constant, K-e, was measured using the NIST Large Agglomerate Optics Facility (LAOF) for soot produced from acetylene and ethene flames. Measurements were performed simultaneously using light sources at 632.8 and 856 nm. The experiments at 856 nm represent the longest wavelength for which accurate extinction measurements have been performed for soot. The mean values of present measurements of K-e at 632.8 nm for the acetylene and ethene soot are 8.12 +/- 0.59 and 9.65 +/- 0.54, respectively. For acetylene, the mean Value of K-e measured at 856 nm was 8.83 +/- 0.69, whereas the mean value for ethene at the same wavelength was 9.35 +/- 0.51. The reduction in discrepancy for the fuels between 632.8 and 856 nm may be related to beam shielding effects. As in the case of 632.8 nm, the measured K-e values for 856 nm are significantly larger than values calculated using traditional methods. The present measurements provide a more reliable value of K-e for use in optical-based soot diagnostics.