The diffusion equation complicated by a delay of a concentration flux, J, from the formation of a concentration gradient, partial derivative c/partial derivative x, was formulated in the context of electrochemical measurements. In contrast with the Fick's first law, J = D partial derivative c/partial derivative x, the flux at a short time is known to be delayed owing to a finite propagation speed of the gradient, called the memory effect or the second sound for thermal diffusivity. The modified Fick's law contained the second time-derivative of the concentration multiplied by the relaxation time, T, additive to the conventional diffusion equation. It was applied to chronoamperometry. The current-time curve was smoother than that for the Cottrell equation. The current at a short time was almost constant owing to the rate-determining step of the propagation velocity, (D/tau)(1/2), and then decays obeying the Cottrell equation. This variation was similar to the curve mixed with the Butler-Volmer kinetics. The relaxation time was estimated from the period during which a diffusing particle can recognize the concentration gradient by collision with the nearest diffusing particle. The propagation velocity was of the order of some cm s(-1), which is similar to the maximum values of the measurable charge transfer rate constant. (c) 2006 Elsevier B.V. All rights reserved.