<para> The modal gain, modal loss and spontaneous emission of a GaAsSb-based type-II quantum-well (QW) laser structure emitting at 1.3 have been experimentally determined as a function of current injection and temperature. The system is able to provide a maximum of 900 of material gain from the transition despite an electron–hole overlap of 32%, however, the gain from the transition becomes dominant before this value can be achieved. The presence of the transition has a detrimental effect on device performance, limiting the usable gain from the first transition and increasing the total radiative recombination current. Energy level calculations show that reducing the hole QW to 4 nm would increase the separation of the and transition by a further 45 meV, reducing the limiting effect of the transition. Carrier distribution spectra show the carriers are in thermal equilibrium for the temperatures and injection currents studied. A low radiative efficiency for this structure is measured due to a very large nonradiative current. We believe a combination of different mechanisms contribute to the nonradiative current. </para>