The molecular structure and composition of gaseous oxalyl fluoride ( OXF) has been investigated by electron diffraction ( GED) at nozzle-tip temperatures of - 10, 149, and 219 degrees C. The GED data were augmented by molecular orbital calculations, and the analysis was aided by use of rotational constants from microwave ( MW) spectroscopy. As in the other oxalyl halides, there are two stable species, of which the more stable is periplanar anti ( i. e., trans). However, unlike these other halides in which the second form is gauche, the second form of oxalyl fluoride was known from MW work to be periplanar syn ( i. e., cis). Our results are consistent with a mixture of trans and cis forms, and yield values for the structural parameters, the composition of the system at the three temperatures cited, and the thermodynamic quantities Delta G degrees, Delta H degrees, and Delta S degrees for the reaction trans -> cis. Some trans/cis distances ( r(g)/angstrom) and angles ( angle(alpha)/deg) at - 10 C are r( C=O)) 1.178-( 2)/ 1.176( 2), r( C-F)) 1.323( 2)/ 1.328( 2); r( C-C)) 1.533( 3)/ 1.535( 3), angle( C-C=O)) 126.4( 2)/ 124.2( 2), D( C-C-F)) 109.8( 2)/ 112.2( 2), and angle( O-C-F)) 123.8( 2)/ 123.6( 2). The mixture compositions ( percent trans) at - 10 degrees C/149 degrees C/219 degrees C are 75( 3)/ 58( 7)/ 52( 8), from which Delta H degrees and Delta S degrees are found to be 1.14 kcal/mol and 2.12 cal/( mol(.)deg). The system properties are discussed.