The calculation of spontaneous electroluminescence from an GaInAsP active layer and the spectral modification of the emitted radiation due to the propagation to the edge of a GaInAsP/InP heterostructure diode is presented. By using the exact solution of a lossy dielectric waveguiding structure, the gain/absorption coefficients calculated for relevant layers of the heterostructure have been transformed into effective absorption coefficients assigned to guided modes. The relevant modes that could propagate through the heterostructure diode have been calculated numerically and the complex effective index of refraction has been evaluated for all existing guided modes. The observed double peak spectra of edge emitted electroluminescence are interpreted in terms of fundamental and higher-order guided modes.