Prompted by interest in reaction-diffusion waves in arsenous acid-iodate and Belousov-Zhabotinsky systems, mutual diffusion coefficients (D) are measured for binary aqueous solutions of arsenous, arsenic, and malonic acids at 25 degrees C and concentrations from (0.00012 to 0.020) mol center dot dm(-3). Arsenous acid, a relatively weak acid (pK(1a) 9.2), diffuses in molecular form in this composition range with D approximate to 1.11 center dot 10(-5) cm(2)center dot s(-1). Arsenic and malonic acids (pK(1a) 2.3 and 2.8, respectively) dissociate appreciably, causing D to increase sharply with increasing dilution due to the unusually large mobility of aqueous H+ ions. Differential D values for solutions of these acids are determined by extrapolating diffusion coefficients measured from Gaussian dispersion peaks to zero concentration difference between the injected solution and the flow solution. Using a simple modification of conventional dispersion equipment, differential diffusion coefficients are measured directly from small-Delta c error-function dispersion profiles generated from step-function initial conditions. Analysis of the concentration dependence of D, including electrophoretic and activity coefficient terms, provides estimates of the diffusion coefficients of the molecular and ionic forms of the acids. The integral mutual diffusion coefficient defined by c(-1)f(0)(c)D(c)dc is identified as the concentration-weighted average of the diffusion coefficients of the molecular and ionic forms of the acids.