Interdiffusion (mutual diffusion) coefficients have been measured by Gouy interferometry for the system HCl + H2O at 25 degrees C from 0.100 M to 11.991 M concentration. They vary from 3.336 x 10(-9) m(2) s(-1) at infinite dilution, to a minimum of about of 3.03 x 10(-9) m(2) s(-1) near 0.1 M, to a maximum of about 6.0 x 10(-9) m(2) s(-1) near 7.25 M, and back down to 4.306 x 10(-9) m(2) s(-1) at 11.99 M. Thermodynamic diffusion coefficients have been calculated and appear to decrease monotonically and smoothly from 3.3368 x 10(-9) m(2) s(-1) at infinite dilution to a value of 0.947 x 10(-9) m(2) s(-1) at 11.99 M. Densities of HCl solutions have been measured from 0.125 M to 12.186 M at 25 degrees C. These increase approximately linearly with molarity M from the density of pure water of 0.997 045 g cm(-3) to a density of 1.1827 g cm(-3) at 12.186 M.