In this paper we report femtosecond studies of the influence of rare-gas collisions on the dissociation reaction of NaI molecules by investigating the coherent wave packet motion in NaI at a temperature of 670 degrees C and rare-gas pressures ranging from 0 to 1000 bar. The photodissociation dynamics of NaI is influenced by the interaction between the excited covalent and the ionic ground state potential energy surfaces, which cross at a certain internuclear separation. Due to the adiabaticity of the so formed potential, studies of the elementary nuclear motion and reaction dynamics along the covalent (Na + I) and the ionic (Na+ + I-) channels are possible. The probing is made for the transition-state complex [Na ... I](double dagger)*. Even at low rare-gas pressures a considerable influence of collisions on the wave packet motion within the adiabatic potential well is found. Only few collisions between NaI molecules and surrounding rare-gas atoms result in fast vibrational relaxation and loss of coherence. Furthermore, a stabilization of the transition state is observed due to the decrease of the Landau-Zener escape probability. The reaction cross section for the interaction of the transition state molecules with helium atoms is found to be smaller than for the interaction with argon atoms. The intensity of the detected laser-induced fluorescence arising from the transition Na <-- Na* decreases with increasing rare-gas pressure. Here, the effect for argon atoms is smaller than for helium atoms.