Incorporating ions within electrodeposited polymer dielectric films creates ultrathin solid electrolytes on a length scale (<50 nm) that bridges molecular electronics and conventional electrochemical devices. Electrooxidation of phenol in basic acetonitrile generates electrodeposited nanoscale (21 +/- 2 nm), pinhole-free poly(phenylene oxide), PPO, films on indium-tin-oxide substrates. Solid-state electrical measurements using top electrode contacts (vapor-deposited An, liquid Hg, or liquid Ga-In eutectic) confirm that these PPO films are electronically insulating (7 +/- 2 x 10(-12) S cm(-1)) with a high dielectric strength (1.7 +/- 0.1 x 10(6) V cm(-1)). The insulating film is converted to an ultrathin solid polymer electrolyte by soaking in a solution of LiClO4-propylene carbonate and then heating under vacuum to remove solvent. Atomic force microscopy establishes that the salt-impregnated film is thicker (43 +/- 10 nm) than the as-prepared PPO film. The X-ray photoelectron spectroscopic measurements suggest minimal retention of solvent in the film. Electrochemical impedance measurements demonstrate that the incorporated ions are mobile in the solid state with an ionic conductivity of 7 +/- 4 x 10(-10) S cm(-1). Such ultrathin solid polymer electrolytes should enable progress toward nanoscopic solid-state ionic devices and power sources.