AbstractThe spectrophotometric determination of the rate of iodine atom catalyzed geometrical isomerization of diiodoethylene in the gas phase from 502.8 to 609.1°K leads to a rate constant for the bimolecular reaction between I andtrans‐diiodoethylene of logkt−c(M−1sec−1) = 8.85 ± 0.12 − (11.01 ± 0.30)/θ. Estimates of the entropy and enthalpy change for the addition of I atoms totrans‐diiodoethylene (process a.b) lead to logKa.b(M−1) = −2.99 − 4.0/θ, and thus to logkc(sec−1) = logkt−c– logKab= 11.8 −7.0/θ for the rate constant for rotation about the single bond in the adduct radical. The theory for calculation of the rotation rate constant is presented and it is shown that while the exact value depends on the barrier height, a value of 6.8 kcal/mole for this quantity leads to logk(sec−1) = 11.8 −6.7/θ. The activation energy points to a better value of the group contribution to heat of formation of the groupC‐(I)2(H