Infrared reflectance spectra of ethyl formate on Ni(111) at 105 K were recorded as a function of surface coverage. The RAIRS activities of several bands are found to vary in a correlated manner as a function of coverage. The activity of both the asymmetric stretching vibration and the twisting vibration of the CH2 group at low coverages conclusively show that adsorption occurs initially in the s-cis,gauche conformation. These two bands fade out on increasing the coverage and are completely removed at full monolayer coverage. The coverage dependence is interpreted in terms of rotational isomerization to a planar conformation as the packing density in the adlayer is increased. The results are consistent with an attribution of the full coverage molecular structure to the s-trans,trans conformation.