Electrochemistry was combined with surface-plasmon-enhanced diffraction (ESPD) to investigate a redox-switchable polymer grating and its (bio-)sensing applications. Patterned arrays of polyaniline (PANI)/ poly(styrenesulfonate) (PSS) were fabricated by the combination of electropolymerization and micromolding in capillaries (MIMIC) and were used as an optical grating for surface-plasmon-enhanced diffraction experiments. The diffraction efficiency (DE) could be tuned by changes in the applied potential, and by changes in the pH of the surrounding solution (dielectric medium). The response of the DE to the pH depends strongly on the redox state of the PANI/PSS grating. If the polymer grating is mainly in its reduced state, the DE shows a linear dependence on the pH. The DE of the PANI/PSS grating can also be modulated by an electrocatalytic event: by keeping PANI/PSS in its oxidized form, the addition of P-nicotinamide adenine dinucleotide (NADH) increases the DE with the increase of NADH concentration, which points to the possibility of the use of ESPD technologies for biosensing.