Gas-phase absorption spectroscopy using a 3.39 mu m He-Ne laser enables the concentration of methanol vapor in a vapor-fed direct methanol fuel cell (DMFC) to be measured in real-time and in situ once high noise levels of the laser are corrected. The changes that methanol vapor concentrations bring to cell performance can be measured under various galvanostatic and potentiostatic conditions and analyzed with regard to methanol crossover and methanol transport capability. The absorption coefficient of +methanol vapor, 0.005336 cm(-1) Torr(-1), is nearly constant under the DMFC operating conditions at a wavelength of 3.39 mu m.