By using a newly developed pulsed plasma probe, direct measurements of electron temperature were performed within mid-latitude sporadic E layers above White Sands, New Mexico. The probe's capability for unfolding density variations from its current voltage characteristic made the temperature measurements possible within the steep density gradients characteristic of Es layers. The major feature of the observations was the presence of an electron temperature gradient across the layers: the bottomsides of the layers were isothermal at Te=340¿K, while the topside of each layer showed Te increasing from 380¿ to more than 500¿K over a vertical extent of 400 m. The results are discussed in terms of reversible heating by gravity waves as well as in terms of plasma density and ion composition effects within the ionospheric plasma itself.