Neutral composition waves with a wavelength of about 5000 km and a wave period of about 2.5 hours were observed by Esro 4 in the altitude region of 250 km. The amplitudes are of the order of 25% for Ar, 15% for N2, and roughly 10% for He and O at 25¿ geographic latitude. The Ar and N2 waves are almost in phase, whereas He is in antiphase, and O is in between. The wave amplitudes are seen to decrease toward lower latitudes, suggesting that the composition waves are launched by auroral sources. Simultaneous ground-based ionosonde measurements of the F2 layer critical frequency at middle and low latitudes show a wave period consistent with the satellite observations. From the relation between wavelength and wave period these waves are identified as gravity waves. The theoretical investigation with a multicomponent model shows that diffusion plays a major role in explaining the observed phase and amplitude relations between atmospheric constituents.