In this work, we measured the densities of 20 protic ionic liquids (PILs) based on four different cations: diethylammonium, triethylammonium, dimethylethylammonium, and triethanolammonium, and 14 anions of the following acids: phosphoric, acetic, trifluoroacetic, methanesulfonic, trifluoromethanesulfonic, benzoic, salicylic, propanoic, sulfuric, nitrobenzenesulfonic, sulfobenzoic, salicylic, sulfosalicylic, and bis(trifluoromethylsulfonyl)imide ones. The values of density were measured over the temperature range from 293.15 K to 353.15 K by densimeter based on the vibrating tube principle and they were approximated by a polynomial as a function of temperature. The thermal expansion coefficients were calculated; the fractional free volume in the molar volume of the studied PILs was estimated, and the influence of the ion structure on their volumetric properties was analyzed. Two different group contribution methods were analyzed to determine their applicability to predicting the density of the studied PILs.