We present a method to retrieve neutral thermospheric composition and the solar EUV flux from ground-based twilight optical measurements of the O+(2P)7320 ¿ and O(1D) 6300 ¿ airglow emissions. The parameters retrieved are the neutral temperature, the O, O2, N2 density profiles, and a scaling factor for the solar EUV flux spectrum. The temperature, solar EUV flux scaling factor, and atomic oxygen density are first retrieved from the 7320-¿ emission, which are then used with the 6300-¿ emission to retrieve the O2 and N2 densities. The retrieval techniques have been verified by computer simulations. We have shown that the retrieval technique is able to statistically retrieve values, between 200 and 400 km, within an average error of 3.1¿0.6% for thermospheric temperature, 3.3¿2.0% for atomic oxygen, 2.3 ¿1.3% for molecular oxygen, and 2.4¿1.3% for molecular nitrogen. The solar EUV flux scaling factor was found to have a retrieval error of 5.1¿2.3%. All the above errors have a confidence level of 95%. The purpose of this paper is to prove the viability and usefulness of the retrieval technique by demonstrating the ability to retrieve known quantities under a realistic simulation of the measurement process, excluding systematic effects. ¿ American Geophysical Union 1994