Heterogeneous electron-transfer rate constants were determined for ferrocene and ferrocene carboxylic acid (FCA) in the room temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), at boron-doped microcrystalline diamond thin-film electrodes. Comparison data for FCA in 1 M KCl were also obtained. The apparent heterogeneous electron-transfer rate constant, k(app)(0), for FCA was 10x lower in the RTIL 1.5 (+/- 1.1) x 10(-3) cm s(-1) as compared to KCl 4.6 (+/- 1.3) x 10(-2) cm s(-1). The k(app)(0) for ferrocene was also 10x lower in the RTIL 5.0 (+/- 1.2) X 10(-3) cm s(-1) as compared to a common organic electrolyte solution 5.5 (+/- 1.2) x 10(-2) cm s(-1). The diffusion coefficient for FCA (D-red) was determined by chronoamperometry to be 1.3 x 10(-7) cm(2) s(-1), ca. 100x lower than the value (1.9 x 10(-5) cm(2) s(-1)) in KCl. The lower diffusion coefficient is consistent with the 100x greater viscosity of the RTIL. The lower k(app)(0) values for these outer-sphere redox systems is attributed, at least in part, to a reduced number of attempts to surmount the activation barrier (i.e., a reduced nuclear frequency factor, nu(n)) due to the more viscous medium. (C) 2013 Elsevier Ltd. All rights reserved.