The structure of ethyl formate was studied by the joint analysis of gas phase electron diffraction, microwave, and infrared data. The experimental data are supported with geometrical constraints and force fields from geometry-relaxed ab initio calculations at the 4-21G and 6-31G** levels. At room temperature the gas phase is found to exist of 61% (sp,ap) and 39% (spl,sc) conformers, corresponding to an Delta H of 0.3 kcal mol(-1). Best geometrical constraints were obtained from 4-21G geometries after correction to r(alpha)(0) level. In contrast, 6-31G**-derived energy differences, rotation barriers, and scaled force fields performed better than their 4-21G counterparts. Assisted by 6-31G**-based IR band intensities, anew assignment of IR frequencies was made. Derived force field constants can be interpreted in terms of bond characteristics and anomeric interactions existing in the rotamers. The force fields in combination with the interpreted IR spectra rationalize why the IR-assisted rotamerization (sp,sc) --> (sp,ap) is about 25 times more efficient than the inverse process (sp,ap) --> (sp,sc) and points to the CH2 rocking and CH2 asymmetric stretching modes as the major contributors.