The spectral linewidth and tuning requirements of an external cavity laser (ECL) in the Littrow architecture and a distributed feedback (DFB) device for space-based gas sensing were investigated. The wavelengths of both devices were in the 935 nm range and different linewidth measurment techniques were explored for the two sources. The linewidth of the ECL was measured using a short-arm homodyne technique, while the linewidth of the DFB was measured with a het- erodyne technique using the ECL as a local oscillator. The spectral linewidth of the ECL was measured to be ,300 kHz with ,5 MHz for the DFB device. The emission wavelength tuning characteristics, including the overall tuning range and the free running stability of the ECL, were also examined. The full frequency drift of the ECL measured over a 36-hour period was found to be approximately 540 MHz, while it showed approximately 0.4 nm continuous mode-hop free wavelength tuning. The objective of this work is the stabilisation of four laser frequencies to four specific predefined wavelengths that are associated with water absorption lines in the 935- 940 nm range for space-based gas sensing. A list of these water absorption lines, with their wavelengths and line- strengths, is shown in Table 1. The experimental set-up involves locking one injection seed laser (ISL) to the stron- gest water line via a water vapour reference gas cell. This laser is then used as a reference to stabilise the other lasers, each to one of the three remaining lines. Locking by means of gas absorption is the preferred technique as the absorption wavelength at a fixed temperature and pressure is constant. The stabilisation technique used for the three remaining lasers employs a Fabry-Perot interfe- rometer to reference from the locked ISL by a fixed detun- ing. The work performed in this research is a portion of a wider study being carried out by the European Space Agency (ESA). The overall project is termed the WALES (water vapour light detection and ranging (LIDAR) exper- iment in space) mission and its objective is to provide