The potential of InGaAsP light-emitting diodes (l.e.d.s) as light sources for quantitative absorption measurements in the 0.9–1.6 μm region is discussed. Since a variety of chemicals, including atmospheric water vapour and pollutants such as CO2 and NO2 have absorption bands in the spectral region spanned by InGaAsP l.e.d.s, this technique is useful in evaluating impurity content. Detection of the atmospheric water vapour band centred at 1.38 μm is demonstrated. The application of this technique for assessing the OH content of optical fibres is also discussed.