Potassium Rydberg atoms are used to investigate the lifetimes, on a picosecond timescale, of transient negative ions formed during dissociative electron attachment to CCl\sb4 and CF\sb3I. The transient negative ions are formed during Rydberg electron transfer reactions of the type$${\rm K}(n{\rm p})+AB\to{\rm K}\sp{+}+AB\sp{-*}\to{\rm K}\sp{+}+A\sp{-}+B$$For low values of principal quantum number n (n \sbsp∼< 20) the electrostatic interaction between the K\sp+−AB\sp−∗ and K\sp++A\sp− ion pairs perturbs the ion trajectories. As a consequence, the final angular and velocity distributions of the K\sp++A\sp− ions are dependent on the lifetime of the AB\sp−∗ intermediate ion. The AB\sp−∗ lifetime is determined by comparing product ion distributions, determined using position sensitive detection techniques, with the results of a Monte Carlo simulation that models the kinematics of the reaction. The data show that the lifetime of the excited CF\sb3Γ\sp∗ intermediate formed by Rydberg electron capture is short, \sbsp∼<2 ps, and that the lifetime of the CCl\sbsp4−∗ intermediate is 7.5±2.5 ps.