The objective of the present work is to experimentally determine the spectral optical constants of two important hydrocarbon fuels, heptane (C7H16) and decane (C10H16). This quantitative information is important for characterizing the absorption behavior of the species, and for analyzing spray combustion processes by laser diagnosis technique. The normal transmittance data of the hydrocarbon species are measured in the wavelength region of 2.5μ to 15μm at various pathlengths using a FT-IR spectrometer. The spectral refractive index, n, and extinction coefficient, k, which give a best fit of analytical trans-mittances to the experimental ones, are tabulated for convenient use.