The density and speed of sound of blends of biodiesel with No. 2 and No. 1 diesel fuels were measured from atmospheric pressure to 32.46 MPa at temperatures of 20 and 40degreesC. The isentropic bulk modulus was calculated from these quantities. The results show that the density and isentropic bulk modulus can be accurately modeled as having a linear variation with blend percentage. Speed of sound is better correlated by a second-order equation. Correlation equations are given and a blending rule is developed that allows the density, speed of sound, and isentropic bulk modulus of blends to be calculated from the properties of the biodiesel and diesel fuel.