The density and refractive index of diluted binary mixtures of active carbon and exfoliated graphite nanoplatelets dispersed in N,N-dimethylformamide were measured at different temperatures, and the ultrasound speed through these materials was determined. From the experimental results, acoustic parameters such as the acoustic impedance, isentropic compressibility, specific refraction, space-filling factor, and relaxation strength were estimated for all compositions and temperatures. The results were used to identify molecular interactions in the mixtures, including structural changes of exfoliated graphite nanoplatelets in polar solvents.