Linear poly(ethylenimine) (-[CH2CH2NH](n)-, LPEI) was modified by attachment of 3-(dimethylferrocenyl)propyl groups to ca. 17% of its nitrogen atoms (FcMe(2)-C-3-LPEI) to form a new redox polymer for use as an anodic mediator in glucose/O-2 biofuel cells. The electrochemical properties of this polymer were compared to those of 3-ferrocenylpropyl-modified LPEI (Fc-C-3-LPEI). When Fc-C-3-LPEI or FcMe(2)-C-3-LPEI was mixed with glucose oxidase and cross-linked with ethylene glycol diglycidyl ether to form hydrogels on planar, glassy carbon electrodes, limiting catalytic bioanodic current densities of up to similar to 2 mA/cm(2) at 37 degrees C were produced. The use of dimethylferrocene moieties in place of ferrocene moieties lowered the E-1/2 of the films by 0.09 V and significantly increased electrochemical and operational stabilities. FcMe(2)-C-3-LPEI was shown to be the more effective polymer for use in biofuel cells and, when coupled with a stationary O-2 cathode comprised of laccase and cross-linked poly[(vinylpyridine)Os(bipyridyl)(2)Cl2+/3+] as a mediator, produced power densities of up to 56 mu W/cm(2) at 37 degrees C. Power density increased to 146 mu W/cm(2) when a rotating biocathode was used. The stability of the biofuel cells constructed with FcMe(2)-C-3-LPEI was higher than that of the cells using Fc-C-3-LPEI. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3505950] All rights reserved.