Product ion yields in postsource decay and photodissociation at 193 and 266 nm were measured for some peptide ions without a basic amino acid residue ([Y-6 + H](+), [F-5 + H](+), and [YPFVEPI + H](+)) generated by matrix-assisted laser desotption ionization (MALDI). Data indicated statistical nature for the dissociation processes. Assuming that peptide ions formed by MALDI are in thermal equilibrium at temperature T and that their dissociation rate constants are specified by the critical energy (E-0) and entropy (Delta S double dagger), a method based on kinetic analysis was devised to determine these parameters simultaneously. The matrix used was found to affect the effective temperature of peptide ions, 2,5-dihydroxybenzoic acid (400-430 K) < sinapinic acid (440 K) < alpha-cyano-4-hydroxycinnamic acid (460-510 K), in agreement with previous perceptions. ED of around 0.6 eV and Delta S-double dagger of -24 eu were smaller than previous quantum chemical results for small model peptide ions.