High level ab initio and CASSCF calculations on the transition structure for the peroxyformic acid epoxidation of ethylene have been carried out to distinguish between a spiro versus a planar orientation of the peracid. The optimized spiro CASSCF (12,12)/6-31+G(d,p) transition structure (Figure la) is a first-order saddle point that is 4.0 kcal/mol lower in energy than the corresponding planar TS after correction for dynamic correlation [CASSCF(MP2)]. The planar TS is 11.5 kcal/mol higher in energy than an unsymmetrical spiro TS. A RSPT2 (6,6)//CAS(12,12)/6-31G(d) correction also favors the spiro TS by 5.3 kcal/mol. Single-point calculations on the spiro and planar CASSCF (12,12)/6-31+G(d,p) transition structures at the UB3LYP, UQCSD(T)and UBD(T) levels favor the spiro symmetrical TS by 9.0, 7.9, and 5.4 kcal/mol [/6-31 +G(d,p)]. The objective choice of the active space is demonstrated to be critical to the transition structure obtained.