Reliable stabilization of the pharmaceutical preparation and the active ingredient remains one of the most important problems of world pharmacy because pharmaceutical preparations are not systems which are stable without limitation. The patient must receive a quality drug and that is why the question of stability is paid grest attention to not only in research and development, industrial manufacture, but also in distribution. The measure of stability is the expiration period. Diluted solution of hydrogen peroxide (3% solution) still belongs to the most widely used and at the same time the most easily accessible disinfectants. In practice it is common both in Slovakia and abroad. It is used in several concentrations. One of its most important disadvantages is its limited stability, which markedly decreases its expiration period. The present paper investigates the stability of hydrogen peroxide solutions of routinely used concentrations (3%, 6%, and 10%) without and with a stabilizing additive (phenacetin) prepared in the pharmacy and stored under different conditions for the period of their expected usability. The content of hydrogen peroxide was assayed by the pharmacopoeial method in 7-day time intervals. All concentrations of 3%, 6%, and 10% hydrogen peroxide were found to fulfil the conditions for stability in the period of time under study. Their concentration did not fall below the limit od 90% of the content of the active ingredient, and storage under decreased temperature proved to be more suitable. Storage of hydrogen peroxide in the light is inadmissible. When the conditions of storage are observed, the required therapeutic effect of hydrogen peroxide solution can be expected for the period of three months.