An important characteristic of wavelength tuneable laser transmitters is that as they tune between output wavelength channels they may generate light at a range of other wavelengths. This effect may ultimately influence the design of WDM wavelength packet-switched networks employing wavelength tuneable transmitters. We have investigated this effect by examining the BER transmission performance of a WDM channel as a function of the degree of attenuation of another WDM wavelength signal during fast wavelength switching events. Our results show the importance of attenuating the output signal from the wavelength switched laser in order to prevent performance degradations on the monitored data channel.