Two laser-based diagnostic methods, resonance-enhanced multiphoton ionization (REMPI) and laser-induced fluorescence (LIF), were used for relative concentration measurements of seven species, O, H, OH, CH, CO, HCO, and CH3, in low pressure stoichiometric methane/oxygen, ethylene/oxygen, and ethane/oxygen premixed hydrocarbon flames. For the well-understood methane/oxygen flame, measured relative density profiles for these seven species were in good agreement with flame modeling calculations. This agreement justifies the use of the stoichiometric methane/oxygen flame as a standard of comparison by which relative density profiles, measured for other hydrocarbon flames, may be placed on an absolute density scale. This approach was applied to measurements for the ethylene/oxygen and ethane/oxygen flames. Peak HCO densities for these flames appear to be overestimated by model calculations based on current kinetic models of C1 and C2 chemistry.