A midinfrared absorption sensor for crank-angle-resolved in-cylinder measurements of gasoline concentration and gas temperature for spark-ignition internal-combustion engines is reported, and design considerations and validation testing in the controlled environments of a heated cell and shock-heated gases are discussed. Mid-IR laser light was tuned to transitions in the strong absorption bands associated with C—H stretching vibration near $3.4\mathrm{\mu m}$ , and time-resolved fuel vapor concentration and gas temperature were determined simultaneously from the absorption at two different wavelengths. These two infrared laser wavelengths were simultaneously produced by difference-frequency generation, which combines a near-IR signal laser with two near-IR pump lasers in a periodically poled lithium niobate crystal. Injection current modulation of the pump lasers produced intensity modulation of the mid-IR, which allowed the transmitted signals from the two laser wavelengths to be detected on a single detector and separated by frequency demultiplexing. Injection current modulation produced a wavelength modulation synchronous with the intensity modulation for each of the laser wavelengths, and accurate measurement of the gasoline absorption signal required the effects of wavelength modulation to be considered. Validation experiments were conducted for a single-component hydrocarbon fuel (2,2,4-trimethyl-pentane, commonly known as iso-octane) and a gasoline blend in a heated static cell ( $300\le T\le 600\mathrm{K}$ ) and behind planar shock waves ( $600<T<1100\mathrm{K}$ ) in a shock tube. With a bandwidth of $10\mathrm{kHz}$ , the measured fuel concentrations agreed within 5% RMS and the measured temperature agreed within 3% RMS to the known values. The $10\mathrm{kHz}$ bandwidth is sufficient to resolve 1 crank-angle degree at $1600\mathrm{RPM}$ .