Extraction of perchloric acid by the nitrobenzene solution of Slovafol 909 (p-nonylphenylnonaethylene glycol, L) has been investigated and the composition of species in the organic phase has been determined. It has been found, that perchloric acid is full dissociated in both phase, and H + ion in the organic phase is partially solvated by one or two molecules of p-nonylphenylnonaethylene glycol. Extraction and stability constants of HL + and HL + 2 species in the organic phase has been evaluated.