Laser flash photolysis studies of the temperature dependence of the rate of escape of oxygen, methyl isocyanide, and tert-butyl isocyanide from horse heart myoglobin (HHMb) and the rate of escape of oxygen from sperm whale myoglobin (SWMb) are presented for neutral pH and the temperature range: 5-40 °C. Enthalpies, entropies, and Gibb's free energies of activation for ligand escape are derived. The room-temperature entropies of activation are small in magnitude:-2.2 to +5 eu. The activation energies, Ea are largest for the alkyl isocyanide ligands: +8.8 and +8.2 kcal/mol for escape of methyl and erf-butyl isocyanide, respectively. For O2, the Et values are slightly smaller: +5.9 kcal/mol, HHMbO2 in 0.1 M bis-tris/0.1 M NaCl buffer; +8.1 kcal/mol, HHMbO2 in 0.1 M phosphate buffer; +7.4 kcal/mol, SWMbO2 in 0.1 M. bis-tris/0.1 MNaCl buffer; and +7.8 kcal/mol, SWMbO2 in 0.1 M phosphate buffer. The Gibb's free energies fall into the range +6 to +7.5 kcal/mol, with the values for the four oxymyoglobin-buffer systems tightly grouped about 7.4 kcal/mol. Multiexponential nanosecond geminate rebinding of oxygen is resolved for all four oxymyoglobin-buffer systems at temperatures below 10 °C. For HHMbO2 in 0.1 M bis-tris/0.1 M NaCl buffer, multiexponential rebinding is observed at temperatures up to 25 °C. Three five-state models are treated in simple numerical simulations of the room temperature multiexponential rebinding time course for HHMbO2. The data cannot be fit to a sequential barrier model but can be fit to either of two other models. One incorporates ligand migration between the usual heme pocket and another adjoining pocket that traps the ligand for some time before permitting it to return and exit from the protein by way of the usual gate. The other considers two distinct escape routes through the protein. © 1990, American Chemical Society. All rights reserved.