Measurements of fuel mole fraction were taken in a model scramjet cavity flameholder using nanosecond-gated laser-induced breakdown spectroscopy with direct spectrum matching. A fuel surrogate, carbon dioxide (CO2), was used to conduct statistical measurements at locations which the laser breakdown initiated sustained combustion in the model scramjet. Verification and calibration testing was performed in a variable pressure combustion chamber to create a direct spectrum matching calibration matrix and confirm the suitability of CO2 as a surrogate for ethylene (C2H4) fuel testing. CO2 was shown to be a good surrogate for ethylene in the scramjet cavity, both in matching fuel/surrogate mole fraction and gas density for conditions with sufficiently high mole fraction. The use of CO2 allowed for statistical measurements of fuel surrogate mole fraction at locations within the cavity that were ignited by the laser spark.