AbstractWe have measured the self‐diffusion coefficient, D, of serum albumin (by the open‐ended capillary method) and the mutual diffusion coefficient, Dm, for serum albumin solutions (by the capillary method, and in the ultracentrifuge) at concentrations up to 0.2 g°Cm−3. Our self‐diffusion data follow those of Keller et al. [J. Phys. Chem., 75, 379 (1971)], but we can find no experimental support for their contention that Dm= D at high concentrations. Indeed, our results closely follow the theoretical relation Dm= D (1 – cv)(1 + 2A2Me +…).Where c, v, A2, and M are, respectively, the concentration, partial specific volume, second virial coefficient, and molar mass of serum albumin.We have also studied the concentration dependence of the frictional coefficients in diffusion and sedimentation. The two frictional coefficients are equal only in very dilute solutions of s