Diode-pumped Nd3+- and Yb3+-doped multimode fibre lasers with external cavities allow sensitive measurements of intracavity absorption in the spectral ranges of 0.9–0.945 μm and 1.03–1.15 μm. The sensitivity is characterized by the effective absorption path length and is derived from the spectral dynamics of the lasers with intracavity atmospheric absorption, and calibrating the emission spectra with HITRAN database. The highest sensitivity is achieved with a cw Nd3+-doped fibre laser; it corresponds to a 140 km effective path length of absorption. Since the achieved sensitivity decreases only weakly upon the pump rate, it seems to be restricted by the joint action of Rayleigh scattering and nonlinear mode coupling. Both multimode fibre lasers may be made the base of sensitive portable multicomponent gas detectors.