We have found that the energy splitting between peaks in the linearly polarized emission from the cleaved surface of an InAs/ GaAs monolayer structure triples with increasing temperature in the range from 5 to 150 K. For each polarization the main emission line corresponds to the radiative recombination of either heavy or light-hole excitons bound to the monolayer. The striking temperature behavior of the peak energies originates from the different hole-phonon coupling due to the much larger penetration of the light-hole envelope function into the GaAs. We prove this assertion by confining the light holes to the InAs plane with a strong magnetic field, which leads to a reduction of the temperature dependence of the heavy-light hole splitting. q 2000 Elsevier Science Ltd. All rights reserved.