Diffusion coefficient is an important property in chemical industry and precise measurements can be achieved by electrochemical techniques. Study of ferrocene diffusion was carried out in 1-hexyl-3-methyl imidazolium hexafluorophosphate-dense CO2 ([C6MIM][PF6–CO2]) biphasic system using microelectrode technique. Diffusion coefficients were determined by cyclic voltammetry and Randles–Ševčík relationship in the temperature and pressure ranging from 303.15 to 333.15 K and 1–10 MPa, respectively. Computed phase simulations were also used. Two-phase system was determined whatever experimental conditions and composition tested. Volumes of heavy and light phase were estimated as well. Both electroanalytical and computed studies showed that [C6MIM][PF6]–CO2 biphasic system containing initial molar fraction of CO2 up to 0.9 can be used without decrease in diffusion coefficient values. The order of magnitude of these diffusion coefficients of ferrocene in greener aprotic media is about 10−6 m2 s−1. Theoretical analysis of ferrocene mass transport was also carried out using Sutherland formula and viscosity model based on Eyring's absolute rate theory. Dramatic decrease in [C6MIM][PF6]-CO2 mixture viscosity with x(CO2) was estimated by calculations.