The collisional relaxation of highly excited molecules plays a very important role in many chemistry processes. The work presented in this thesis studies the collisional quenching dynamics of highly vibrationally excited molecules using high–resolution transient IR absorption spectroscopy. This work investigates “weak” (small energy transfer) and “strong” (large energy transfer) collisions between donor and bath molecules. The experimental results illustrate how the properties of donor molecules influence the collisional energy transfer. These properties include the molecular structure, internal energy, state density. In several weak collision studies, this thesis studies the vibration–rotation/translation pathway for pyrazine/DCl, pyrazine/CO₂ with different internal energies and for three excited alkylated pyridine molecules/CO₂ systems. A single–exponential rotational distribution and J–dependent translational energy distributions of scattered DCl molecules are observed. For CO₂ collisions, the scattered CO₂ has a biexponential rotational distribution and J–dependent translational energy distributions for all collision pairs. Recoil velocities scale with product angular momenta. The observed collision rates for these collision pairs match Lennard–Jones rates. The full energy transfer distribution for these pairs is determined by combining data for weak and strong collisions. Lowering the internal energy of donor molecules reduces the amount of rotational and translational energy transfer to CO₂. Reducing the internal energy of pyrazine decreases the probabilities of strong collision and increases the probabilities of weak collision. The average energy transfer reduces by ∼ 50% when the internal energy is decreased by only 15%. The collision rates are independent on the internal energy for these systems. Methylation of donor molecules decreases the magnitude of V—RT energy transfer. The collision results are affected by the number of methyl–groups, and not by the position of the group. Increasing the number of methyl groups increases the ratio of the measured collision rate to the Lennard–Jones collision rate. In the strong collision studies, the effects of alkylation and internal energy are studied. In collisions with alkylated pyridine donors with E ∼ 39000 cm−1, CO₂ molecules gain less energy from alkylpyridine than from pyridine. The alkylated donors undergo strong collisions with CO₂ via a less repulsive part of the intermolecular potential compared to pyridine. For azulene/CO₂ collisions with two different internal energies, scattered CO₂ molecules gain double the amount of rotational and translational energy when the azulene energy is doubled. The rate of strong collisions increases four times when the internal energy is doubled.