Densities of dilute aqueous solutions of thiourea, 1,3-dimethylurea, and 1,3-dimethylthiourea were obtained using the Anton Paar DMA 5000 M vibrating-tube densimeter. The apparent molar volumes and expansibilities (down to infinite dilution) were computed from the measured density data. The experiments were performed at five temperatures from (278.15 to 318.15) K and atmospheric pressure. A comparison of volumetric and some other thermodynamic changes caused by the N,N'-methylation in thiourea and urea molecules are discussed.