The application of wavelength modulation absorption spectroscopy in the characterization of lineshape functions of molecular spectra is discussed theoretically, and experimental results for the weak spin-forbidden, magnetic dipole-driven transitions of the oxygenAband are provided. Factors affecting the accuracy of the method are discussed, and it is shown that the technique allows one to accurately probe absorption lineshape functions by using a simple, cost-effective apparatus. The advantages of using detection harmonics greater than the commonly used second are discussed, including an investigation of wing structures of absorption lines and the measurement of collision cross sections. ©1997 American Institute of Physics.