In order to understand the strength and type of interactions involved in ternary solutions, the effect of solute or cosolute concentration and temperature is needed, as an understanding of these interactions is helpful in biochemical and biophysical chemistry. In this regard, we have studied the volumetric and ultrasonic properties of one of the most important vitamins, i.e., ascorbic acid, in water and in the presence of newly synthesized protic ionic liquid (1,8-diazabicyclo[5.4.0]undec-7-en-8-ium trifluoroacetate) at temperatures of 293.15–328.15 K and at atmospheric pressure. The experimentally measured density and speed of sound data were used to calculate apparent molar volume and isentropic compressibility, infinite dilution partial molar volume, and partial molar isentropic compressibility. Volume of transfer (ΔtV2° and ΔtK°s,2), expansion coefficients, pair and triplet volumetric interaction coefficients were also evaluated and discussed in terms of various interactions occurring between ascorbic acid and PIL on the basis of the structural interaction model.