A thermochemical study of the protic ionic liquid 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]), a prospective cellulose solvent considered for the Ioncell-F process, was carried out. The heat capacities of 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and [DBNH][OAc] were measured by differential scanning calorimetry (DSC) at 223-323 and 273-373 K temperature ranges, respectively. The enthalpies of fusion and synthesis reaction of [DBNH][OAc] were measured by DSC and reaction calorimetry, respectively. The gas-, liquid-, and solid-phase enthalpies of formation of [DBNH][OAc] and DBN were determined using calorimetric and computational methods. The enthalpy of vaporization of [DBNH][OAc] was estimated from the formation enthalpies. The activity coefficients at infinite dilution of 17 and the enthalpies of solution at infinite dilution of 25 organic solutes in [DBNH][OAc] were measured by gas chromatography and solution calorimetry methods, respectively. The obtained data will be used in the design and optimization of the Ioncell-F process.