The paper presents studies of the transport and electrochemical characteristics of solid composite electrolytes (1 – x)(C₂H₅)₃CH₃NBF₄–xC_ND (where C_ND is the UDA-С nanosized diamonds with S_s = 300 ± 20 m²/g, 0 < x < 1). The addition of a nanodiamonds inert additive is shown to lead to an increase in the electrical conductivity of the composite electrolyte by 2 orders of magnitude up to 1.7 × 10^–3 S/cm at 200°C for x = 0.98. The theoretical dependences describe well the experimental data over the 0 < x < 0.98 concentration range at temperatures of 25–200°C. The stability of composite materials in a C/0.6(C₂H₅)₃CH₃NBF₄–0.4C_ND/C cell was studied by cyclic voltammetry. The fundamental possibility of using the composite solid electrolytes with nanodiamonds additives in electrochemical devices is substantiated by the example of a C/0.6(C₂H₅)₃CH₃NBF₄–0.4C_ND/C solid-state supercapacitor. Thus, it was demonstrated that nanodiamonds can be considered as an effective non-oxide additive in composite solid organic electrolytes used in electrochemical devices.