A simple low-cost experiment has been developed for the measurement of the binary diffusion coefficients of liquid substances. The experiment is suitable for demonstrating molecular diffusion to small or large undergraduate classes in chemistry or chemical engineering. Students use a cell phone camera in conjunction with open-source image processing software to measure concentrations of a colored species in a clear liquid, as a function of position and time. Three fundamental principles in mass transfer and spectrophotometry are demonstrated: Fick's first and second laws of diffusion and the Beer-Lambert law for absorption of light. The measured value for the binary diffusion coefficient for potassium permanganate in water using this method was found to be within 10% of literature values at the 95% confidence interval.