The paper presents studies of the transport and electrochemical characteristics of solid composite electrolytes (1 – x)(C2H5)3CH3NBF4–xCND (where CND is the UDA-С nanosized diamonds with Ss = 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)3CH3NBF4–0.4CND/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)3CH3NBF4–0.4CND/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.