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We report a systematic study of the polarization switching behavior during single mode emission in gain-guided VCSELs. An explanation for polarization switching in VCSELs has been offered based on the fact that self heating of the devices by the injected current changes the relative spectral alignment between the laser gain and the cavity resonances of nondegenerate polarization states. A theoretical model including magnetic sub-levels of the conduction and heavy-hole valence bands in a quantum-well VCSEL predicts additional polarization switching mechanisms that occur even when the active region temperature is kept constant. We show experimental evidence of such polarization switching at constant active region temperature. The lasers used in this study are proton-implanted AlGaAs/GaAs multiple quantum well VCSELs on a linear array of devices with active region diameter of 15 /spl mu/m and top contact window diameter of 12 /spl mu/m, operating around 850 nm.