Abstract The physical properties of 3-propyl-4-ethylsydnone at various temperatures were investigated by dielectric constant (ε), refractive index (nD), density (ρ), and viscosity (η) measurements. The dielectric constant of 3-propyl-4-ethylsydnone was very high compared with those of many organic solvents. For example, it was found to be 64.6 at 25 °C, close to that of propylene carbonate (64.9), a typical nonaqueous polar solvent with a high dielectric constant. The refractive index and density for 3-propyl-4-ethylsydnone decreased linearly with increasing temperature. The viscosity of 3-propyl-4-ethylsydnone at 25 °C was 11.2 cP (1 cP = 10−3 N s m−2). This was larger than those of many organic solvents and close to that of phenol (11.6 cP at 20 °C). From a line for log η vs. 1/T, the activation energy (Eη) for the viscosity of 3-propyl-4-ethylsydnone was found to be 29.7 kJ mol−1. The estimation of donor (DN) and acceptor (AN) numbers for 3-propyl-4-ethylsydnone were also done from 29Si and 13C NMR spectroscopies, respectively. Donor number of 3-propyl-4-ethylsydnone was about 25, and its acceptor number was approximately about 20. Both numbers were large in many organic solvents. Both physical and donor–acceptor properties were influenced by alkyl substituent groups in the 3-, 4-positions of the sydnone ring.