Theory for the chronoamperometric positive feedback mode of the scanning electrochemical microscope (SECM) is extended to include the situation where the oxidised and reduced forms of the redox mediator couple have arbitrary diffusion coefficients. Under typical positive feedback conditions, the solution initially contains a redox-active species, R, along with excess supporting electrolyte. The potential of the tip ultramicroelectrode (UME), positioned close to an interface of interest, is adjusted to a value where R is electrolysed to produce species O at a diffusion-controlled rate. O diffuses away from the tip towards the interface, where the reverse redox reaction occurs leading to the production, and diffusional feedback of R for electrolysis at the tip electrode. When positive feedback measurements are carried out under diffusion-controlled chronoamperometric conditions, the form of the normalised current-time behaviour, at a particular tip to interface distance, is found to be sensitive to the ratio of the diffusion coefficients of the O/R couple. As a steady-state is established, the normalised current becomes independent of the diffusion coefficient ratio and depends only on the tip to interface distance. Experimental measurements on the chronoamperometric oxidation of ferrocene (Fc) in acetonitrile solution at a Pt tip UME positioned close to a Pt substrate electrode, provide support for the theoretical predictions and demonstrate that the diffusion coefficient ratio can readily be determined from SECM chronoamperometry. Although Fc and Fc(+) are often assumed to have the same diffusion coefficients, steady-state and chronoamperometric measurements at a conventional UME yield a value of 2.0 X 10(-5) cm(2) s(-1) for the diffusion coefficient of Fc, while SECM chronoamperometry indicates that the diffusion coefficient of Fc(+) is 1.6 X 10(-5) cm(2) s(-1).