Polyoxymethylene dimethyl ethers (DMMn), which are ideal additives for diesel fuel, are mainly synthesized from the condensation of methanol (MeOH) or dimethoxymethane (DMM) with 1,3,5-trioxane (TOX) or paraformaldehyde (PF) promoted by different acid catalysts. However, up to date, few studies have been reported to examine the formation mechanism of DMMn which is essential in understanding the reaction and valuable in designing improved catalysts. In this work, using the density functional theory (OFT) calculations combined with experiment studies, we evaluate the formation mechanism of DMMn, which is promoted by sulfonic acid-functionalized ionic liquids (SO3H-FILs). Our calculated results indicate TOX and PF should dissociate into formaldehyde monomers firstly and then to react with MeOH or DMM. However, their decomposition process is different where the dissociation of TOX proceeds along a two-step mechanism while it follows a one-step mechanism for PF dissociation. As for the formation of DMMn, the reaction proceeds along a hemiacetal-carbocation pathway when MeOH is selected as the capping group provider, while the reaction follows a carbocation pathway when DMM is chosen. The origination for the product distribution pattern has also been discussed in detail. The cations and anions of ionic liquids are found synergistically promote the condensation reaction by proton transfer and simultaneously stabilizing the formed intermediated and transition states. Moreover, all the processes related to the decomposition of TOX and PF and the condensation reaction are reversible. (C) 2015 Elsevier B.V. All rights reserved.