Ab initio molecular dynamics simulation is used to investigate the kinetics and thermodynamics of some of the chemical reactions that occur during the induction phase of acid-catalyzed polymerization of 1,3,5-trioxane, In particular, the first ab initio calculation of a free-energy profile in a condensed-phase system is presented. The introduction of an Hf ion to a sample of trioxane liquid initiates the complete protolysis of several trioxane molecules in a rapid succession of picoseconds. Subsequently, the re-formation of small formaldehyde oligomers is observed, which break up again after 1-2 ps. The fast kinetics is found to be consistent with the results of a constrained ab initio molecular dynamics evaluation of the free-energy profile for the formation of a protonated dimer. Hn the trioxane-formaldehyde mixture, this reaction is found to be barrierless with a reaction free energy in the thermal range (IO kJ mol(-1)), Solvation of the chemically active carbocation by formaldehyde molecules reduces the binding energy compared to that in the gas phase by 1 order of magnitude.