A tunable diode laser infrared high-resolution spectrometer was constructed to study the spectroscopy and kinetics of free radicals. A method for producing diode laser frequency scans which are several wavelengths long, linear in frequency, and readily and accurately calibrated from reference spectra has been devised. The laser is current scanned under computer control over short overlapping segments between each of which a temperature step is made. Each segment is then linearized and pieced together to provide a final spectral scan. High-resolution spectra of transient species were collected using either magnetic rotation spectroscopy or flash kinetic spectroscopy. The high-resolution infrared spectrum of the CH stretching fundamental of the formyl radical (HCO) was observed by means of infrared kinetic spectroscopy using 308 nm (XeCl) excimer laser flash photolysis of formaldehyde or acetaldehyde followed by diode or difference-frequency laser probing of the transient absorption. The spectra obtained were assigned and fitted with rotational, spin-rotational, and centrifugal distortion constants resulting in a v\sb1 band origin of 2434.48 cm\sp−1. New ground state constant are reported from a least-squares fit combining the v\sb1 infrared data with previous microwave and far-infrared laser magnetic resonance (FIR LMR) measurements. Fixed-frequency studies of infrared absorptions using the spectrometer allow the time-resolved investigation of chemical dynamics on submicrosecond time scales. A method of locking the diode laser output frequency to an external vacuum-spaced etalon by modulating the optical pathlength of the etalon was investigated. Using this technique, the rate constant of the reaction of HCO with O\sb2 was measured.