The combination of a scanning monochromator, a digitizing oscilloscope and a computer, allows time-resolved spectral measurements on laser diodes. The flexible system described covers a wide range of timescale, well below 1 microsecond(s) ec at a spectral resolution of down to 0.05 nm. With this system the authors have investigated quasi-cw laser diode bars that are designed for solid-state laser pumping, at various operating conditions. The time evolution of the center wavelength is quite well fitted by a square root function according to non-stationary thermodynamics. The measuring system enables judgement of the quality of mounting technique and helps in choosing the right heatsink to minimize the wavelength chirp during a light pulse.