We have directly measured the carrier diffusion length Ld of dilute nitride single quantum wells (QWs) by means of spatially resolved cathodoluminescence (CL) spectroscopy as a function of the CL detection energy (E ) for different temperatures. While Ld (E ) of a GaAs/(Al,Ga)As reference sample does not vary with E , Ld (E ) of (In,Ga)(As,N)/GaAs QWs is significantly reduced with increasing E . The qualitative difference of Ld (E ) between the binary and quaternary QWs is explained in terms of different transport mechanisms (hopping versus tunnelling, respectively) in conjunction with the presence of localized states. Ld of (In,Ga)(As,N) QWs decreases rapidly with increasing temperature. Ld can be significantly reduced by the presence of extended defects even at low temperatures. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)