High-power, high-coherence solid-state lasers, based on dielectric materials such as ruby or Nd:YAG (yttrium aluminium garnet), have many civilian acid military applications. The active media in these lasers are insulating, and must therefore be excited (or ’pumped’) by optical, rather than electrical, means. Conventional gas-discharge lamps can be used as the pumping source, but semiconductor diode lasers are more efficient, as their wavelength can be tailored to match the absorption properties of tbe lasing material. Semiconducting AlGaAs alloys are widely used for this purpose(1,2), but oxidation of the aluminium and the spreading of defects during device operation limit the lifetime of the diodes(3), and hence the reliability of the system as a whole, Aluminium-free InGaAsP compounds, on the other hand, do not have these lifetime-limiting properties(4-8). We report here the fabrication of high-power lasers based on InGaAsP (lattice-matched to GaAs substrates), which operate over the same wavelength range as conventional AlGaAs laser diodes and show significantly improved reliability. The other optical and electrical properties of these diodes are either comparable or superior to those of the AlGaAs system.