A mediator composition for coating an electrode of an amperometric sensor wherein said mediator composition comprises a mono, bis or tris (4,4'substituted 2,2'-bipyridine) complex of a metal M having the following formula (I) ##STR11## in which M represents iron, ruthenium, osmium or vanadium, R and R' are the same and each is a hydroxy or alkoxy group, or a primary, secondary or tertiary amine gr